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The Oxford Handbook of the History of International Law edited by Fassbender, Bardo; Peters, Anne (1st October 2012)

V Methodology and Theory, 42 The Reception of Ancient Legal Thought in Early Modern International Law

Kaius Tuori

From: The Oxford Handbook of the History of International Law

Edited By: Bardo Fassbender, Anne Peters

From: Oxford Public International Law (http://opil.ouplaw.com). (c) Oxford University Press, 2015. All Rights Reserved. Subscriber: null; date: 06 April 2020

Ancient Times to 1648 — Since World War II — Use of force, war, peace and neutrality — High seas

(p. 1012) 42  The Reception of Ancient Legal Thought in Early Modern International Law

1.  Introduction

During the early modern humanistic revival of legal science, scholars sought material and inspiration from classical, mostly Roman, sources. Because generally the reception of law or the transfer of elements of one legal culture to another involves changes in the transferred elements as well as the recipient culture, this reception of ancient law was a complex affair in which not only the content of ancient law but also its cultural prestige were employed to further the agenda of the scholars who utilized it. Furthermore, the transferred law, in the way legal transplants operate, was transformed as well.1

(p. 1013) The purpose of this chapter is to explore the reception of ancient law and legal thinking by early modern scholarship on international law,2 by authors like Alberico Gentili (1552–1608) and Hugo Grotius (1583–1645),3 who were mainly responsible for the first stage of the reception, and later scholars like Samuel Pufendorf (1632–1694), Cornelius van Bynkershoek (1673–1743), and Christian Wolff (1679–1754).4

The issue at hand contains numerous preliminary controversies in need of clarification. The foremost of the issues is whether or not there existed an international law in antiquity which would have been the object of reception in the early modern era. The ancient traditions of international law naturally encompassed much more than the products of the Greco-Roman world,5 but their later influence has been (p. 1014) limited. Greek and Biblical sources were extensively used by scholars in the early modern period, but their impact was less extensive than the influence of Roman law. While the influence of the Greek tradition of international law as manifested in concepts like treaties between sovereign States has seldom been questioned, the same cannot be said of the Roman legal tradition. Though the strong general influence of the Roman legal tradition in the early modern era—be it in general on the European legal traditions, or specifically in the scholarship on international law—is not in doubt, whether or not there was a Roman tradition of international law is debatable.6 However, within the scholarship on, for example, Stoic influence in international law, strict distinctions between traditions, be they Greek or Roman, are not necessary.7

The Romans divided law into ius civile and ius gentium, in which the first pertained to the citizens of Rome, while the second was law applied to the peregrines in Roman courts. Thus the Roman ius gentium was not international law in the current sense but mostly substantive Roman law applied and developed by Roman magistrates with some characteristics resembling private international law.8 The actual law between nations was originally covered by the Roman law of warfare, the ius belli and ius fetiale, which governed the relationship between Rome and its allies and enemies.

Two main interpretations have been made of the impact of Roman law in modern international law. The first is that there was a Roman tradition of international law and that there is continuity from the Roman to the modern tradition. The second, currently more widely accepted interpretation is that the ancient Roman legal tradition that was received and influenced the development of international law in the early modern period was the Roman private law tradition. Naturally, whether there are lines of continuity depends also on the definition of international law.9

(p. 1015) The reuse of ancient law in the early modern era was a product of a number of cultural, political, and social factors. The period was defined by religious polarization and the reduction of the intellectual authority of the church. It has been claimed that, for example, Grotius utilized Roman law to develop a denominationally neutral alternative to Christian ethics as a foundation of natural law.10 There was also a widespread idealization of classical antiquity, classicism, which manifested itself in legal scholarship in the conviction of the superiority of Roman legal science and its use as an example for contemporary jurisprudence.11

This chapter will first explore the general influence of classical culture in the emerging international law scholarship. The first issue is the development of the concept of ius gentium in the Roman legal tradition and its transformation into the concept of international law as currently understood from Roman sources and early modern scholarship. The second question is the use of the classical cultural and literary tradition. The cultural prestige of classics was significant and the utilization of the classical literature in education was extensive. How much did this lead to the reception of ancient practices and ancient law?

A selection of examples of ancient law transplants and their transformation will be followed through some of the classical works of early modern international law.12 The selection of three cases reflecting ancient public international law and three cases showing the transfer of private law conceptions is quite arbitrary and aims only to provide glimpses of a very wide and under-studied field. Many interesting themes are left for future research to tackle.

Of the six practical examples of the transmission of ancient law to early modern international law, the first three are of the more traditional law of nations, while the following three are uses of the Roman private law analogies in international law. How were the Roman conceptions of a just war, the sanctity of envoys, and the freedom of the high seas received in the early modern doctrine of international law? How were analogies from Roman private law such as pacta sunt servanda, terra nullius and uti possidetis utilized to create new rules for the changing circumstances of the early modern era? In all of the examples, it is interesting to follow how the rules established through such reception are retained even after references to the ancient precedents are no longer made.

(p. 1016) 2.  From Roman to Early Modern Ius Gentium

2.1.  Gaius and the Digest

Already in the Roman sources one may see the transformation of the concept of ius gentium from the original Roman understanding of substantive law applied to peregrines to a meaning closer to the idea of the law of nations.13 Marcus Tullius Cicero (106–43 bc) outlines the division between ius civile and ius gentium as two overlapping spheres, with the law of nations deriving from nature and the civil law from human society.14 Although Cicero's conceptions of ius gentium and natural law are by no means clear, ius gentium is defined as part of unwritten law common to all people.15

Gaius’ textbook of Roman law, written in the mid-2nd century ad gives a similar definition, but with greater detail:

All peoples who are governed by laws and customs use law which is partly theirs alone and partly shared by all mankind. The law which each people makes for itself is special to itself. It is called ‘state law’, the law peculiar to that state. But the law which natural reason makes for all mankind is applied in the same way everywhere. It is called ‘the law of all peoples’ because it is common to every nation. The law of the Roman people is also partly its own and partly common to all mankind. Which parts are which we will explain below.16

Gaius’ idea of ius gentium may be described as very cosmopolitan,17 while his near contemporary Aulus Gellius also juxtaposes ius gentium with natural law.18

Ulpian, a jurist writing in the early 3rd century, added natural law as the third component to Cicero's and Gaius’ bipartition: (p. 1017)

Private law is tripartite, being derived from principles of jus naturale, jus gentium, or jus civile. Jus gentium, the law of nations, is that which all human peoples observe. That it is not co-extensive with natural law can be grasped easily, since this latter is common to all animals whereas jus gentium is common only to human beings among themselves.19

Ulpian specifically names slavery and manumissions as institutions of ius gentium,20 while civil law overlaps with both natural law and ius gentium but is special to the Roman people.21 The Constitutio Antoniniana of AD 212 had in theory made all the inhabitants of the Roman empire citizens, effectively making the old division between ius civile and ius gentium redundant except in cases involving peregrines outside the empire.

Hermogenian, a jurist in the late 3rd century, gives a very different definition of ius gentium in a passage wedged between Ulpian's passages in the Digest:

As a consequence of this ius gentium, wars were introduced, nations differentiated, kingdoms founded, properties individuated, estate boundaries settled, buildings put up, and commerce established, including contracts of buying and selling and letting and hiring (except for certain contractual elements established through jus civile).22

It is possible that this radically different definition is a result of a fundamental change in the understanding of ius gentium, but it is also conceivable that it is an extension of earlier practice and that the quotation does not preserve the whole idea. The use of ius gentium in the sense of rules applying between nations may be found in the Roman historical literature already in Livy. In legal writings, the second-century AD author Pomponius called the rules regarding envoys ius gentium, in accordance with the modern usage.23

The new interpretation of ius gentium caught on and towards Late Antiquity its definition as the law between nations becomes dominant. Isidore of Seville's (560–636) definition of ius gentium is again a heterogeneous list of things that most nations have in common:

The law of nations (ius gentium) concerns the occupation of territory, building, fortification, wards, captivities, enslavements, the rights of return, treaties of peace, truces, the pledge not (p. 1018) to molest embassies, the prohibition of marriages between different races. And it is called the ‘law of nations’ because nearly all nations (gentes) use it.24

The important change in Isidore in contrast to Ulpian is that ius gentium is no longer the usages of all nations, but that of almost all nations.25 Also, contracts between individuals, which were included in Hermogenian's definition, were left out. Ulpian's definition reflected the Stoic ideals of universality and equality, whereas Isidore paves the way for what is the realistic and limited interpretation of ius gentium as the practice of civilized nations.

2.2.  Medieval Transformations of Ius Gentium

It was Isidore's definition that was adopted into church doctrine in the Middle Ages. The Decretum Gratiani borrowed extensively from Isidore in its definition of human and natural laws and even incorporated the definition of ius gentium in a slightly altered form.26 Thus Isidore's definition became part of the Corpus Iuris Canonici. The fact that Isidore's definition was adopted took place for reasons that are unclear but probably include textual transmission because Late Antique sources were as a rule better known during the Middle Ages.

Aquinas began as well with Isidore's definition, but categorized ius gentium under natural law, because natural law was common to all humans whereas ius gentium was common only to most nations.27 Here Aquinas combines Isidore with Cicero, another medieval favourite, as well as with Ulpian.

2.3.  Ius Gentium in the Early Modern Understanding

The early modern period is marked by two traditions of interpretation, the medieval Scholastic tradition and the new humanistic school of thought, which favoured (p. 1019) classical over Late Antique sources.28 Vitoria's conception of ius gentium has been described as an intermediary stage between natural law and positive law. It was not dependent on municipal law on its validity, but was based on reason and the common conviction of all peoples.29 Although it is questionable whether Vitoria made a paradigmatic shift from Gaius’ international private law to international interstate law, the conscious ambiguity allowed a wide interpretation that encompassed as subjects both States and people within States.30

Gentili, starting from the antiquarian position, first pays lip service to the laws of the fetiales, which governed Roman conduct on international treaties, war and peace, and embassies. He then notes that these existed during the earlier period of the Roman State and fetial law all but disappeared during the empire. Thus the search for the sources of the law of nations should be directed elsewhere, and here Gentili traces the definitions of ius gentium from the Digest to Baldus, Doneau, Ambrose, St Jerome, and so on. While he recognizes that the common agreement of all nations is the starting point of the law of nations, there is a deeper root than that; namely the innate sense of justice, the instinctive natural desire for justice. As manifestations and sources of this natural justice, sources like the law of Justinian or the Bible are sources of the law of nations. However, it is the community of peoples and its customs which are the most important.31

Grotius defined the law of nations as law that gained its validity from the will of all nations or of many nations. The latter limitation Grotius justified with the claim that there was little law outside the law of nature which would apply to literally all nations. The content of the law of nations may be found in the description of the unbroken custom by expert scholars of history.32

Winkel claims that the change in the definition of ius gentium was fundamental in the reception of Roman private law concepts in the early international law. (p. 1020) The influence of Grotius and of the Westphalian peace treaties is crucial in this transformation, which is parallel to the general reception of Roman law in legal scholarship.33

Earlier, Nussbaum had traced the evolution of the concept of ius gentium from Roman municipal law to international law (in itself a term coined only by Bentham in 1789) to Suarez and Hobbes, in which the first would have distinguished two meanings of ius gentium, universal law and international law, whilst the latter used ius gentium exclusively for international relations.34

The decline of the importance of a classical pedigree is evident in the nascent natural law scholarship. Pufendorf questioned the existence of a law of nations separate from natural law. While he acknowledges that there was a Roman ius gentium which dealt with the rights of non-citizens in Rome, the law of nations which applies between peoples and nations is only a segment of natural law.35

Along with the traditions of natural law scholarship, Wolff dispensed with the niceties of presenting a lineage from the classical tradition and refers only to Grotius and himself. He divided ius gentium into four kinds: voluntary, that is, law which arises from universal consensus; stipulative, or law deriving from pacts between nations; customary, or law based on long usage; and, finally, positive law originating from the will of nations.36

3.  The use of the Classical Tradition in Early Modern International Law Scholarship

3.1.  Gentili

The omnipresence of the classical world in early modern scholarship on international law is a fact easily established by a simple look at any of the works of the major scholars of the era.37 For example, a passage in Gentili's De jure belli on religion (p. 1021) invokes medieval historical figures Dagobert and the Frisians and Charles the Great and the Saracens, but inexplicably moves to Biblical Judith and Holofernes in the next sentence, to ancient history's Cambyses in the following, then to Antiochus, and finally to the Roman Emperor Elagabalus.38 For Gentili, the ancient world was a storehouse of examples of human conduct and even though Biblical and more contemporary examples abound, the Greek and especially Roman examples are closest to his heart. For example, when he writes how barbarians may be treated more harshly in war than civilized peoples, the Roman example of crushing the barbarians while winning the civilized over with kindness is mentioned first and only corroborated with Ambrose reminding how Moses had spared not one of the Midianites.39

Though not a classicist like many of his scholastic contemporaries, Gentili used the corpus of classical literature extensively and usually quoted their opinions with approval. Unlike some later scholars such as Grotius, Gentili eagerly compared ancient and modern policies and courses of action. For example, on the thorny issue of whether a ruler may resort to war to maintain religion among his subjects, Gentili starts down the road of Biblical, Greek, and Roman examples, but dispenses with them early on and gets to the issue regarding Luther and his followers. Though Roman examples are repeatedly cited in favour of religious tolerance and harmony, the argument is aimed at contemporaries. Here the Roman world is mentioned both for its tolerance and intolerance, as is the Turkish realm and Austria.40 In his De armis romanis Gentili addresses the issue of Roman examples and exemplarity for the international law and demonstrates how classical material may be used to argue both sides of an issue.41

3.2.  Grotius

Grotius’ familiarity with the classical literature was profound. An astute classical philologist, he published translations, commentaries, and other works on inter alia Lucan, Plutarch, Euripides, Tacitus, and Seneca.42 Even though posterity remembers him as a legal scholar, he wrote theological treatises, historical works, poetry, political tracts as well as several collections of letters. How this immense knowledge is put to use varies tremendously from text to text and subject matter. For example, when discussing the obligations of kings, he refers to Vásquez and Suarez on the division between the actions of a king as a king and the actions of a king as a normal person, whereas in the next chapter, when he discusses how the king is bound by the law of (p. 1022) nature and not municipal law, the references are to Justinian's Digest and the Code and Baldus’ commentary on them.43 Then again, in De jure belli ac pacis numerous chapters are specifically devoted to how an issue is dealt with in Hebraic law or Roman law.

Besides Grotius’ background as a humanist lawyer, Straumann separates four reasons for Grotius’ use of the Roman law tradition. First, Grotius wanted to provide a secular, denominationally neutral natural law not based on Biblical tradition. Second, Roman law had developed a ready doctrine for the freedom of the seas. Third, Roman sources tended to support imperialism, which could be used to justify Dutch expansion. Finally, there were advanced contract law remedies in the Roman ius gentium which were not limited to Roman citizens. Grotius’ theory of subjective natural rights was thus fundamentally founded on the Roman tradition, not on medieval scholars.44 Grotius was of course not limited to using the Roman tradition; his usage was more instrumental in that he always sought support for his constructions wherever he could find it.45

3.3.  Pufendorf and Wolff

Pufendorf's reliance on the classical world is strong yet not as pervasive as in the case of Gentili or Grotius.46 Classical literature continues to supply most of the examples and bon mots, but the theoretical framework is that of early modern legal and philosophical scholarship. For example, the theory that all men are naturally equal is supported by statements by ancient authors like Statius, Seneca, Vergil, Cicero, Diodorus Siculus, Quintilian, and Euripides, but the argument itself is aimed at Thomas Hobbes and Michel de Montaigne.47

Wolff's relationship to the classical world is very different from the earlier authors as is evident from the almost complete lack of references to classical sources and materials.48 For example, in the Jus gentium almost the only use of classical material is a description of the Roman practice of declaring war.49 It has been argued that while Gentili and Grotius relied extensively on Roman jurists not as authoritative figures but as evidence, the main figures of the natural law school, Wolff and Vattel, were not familiar in Roman law, which led to its waning influence in international law.50

(p. 1023) 4.  Law of Nations: Just War, Sanctity of Envoys, and the Freedom of the High Seas

4.1.  Just War

The clearest description of the Roman just war (bellum iustum) tradition may be found in Cicero's account of the ius fetiale. The procedure was formulated like a trial in court, in which the Romans first presented their demands (rerum repetitio) in which the injuries suffered by the Romans and their claims to reparations were presented to the opposing side. A denuntiatio, or notification of the possibility of war, was issued thirty days later, followed by an indictio, a declaration of war. It should be noted that the historical accuracy of the procedure is controversial and it was used only during the Early Republic.51

Elsewhere, Cicero frequently defends self-help and self-defence as lawful, right, and just. In Pro Milone he famously justifies self-defence as self-preservation when other remedies are not available.52 In the Digest of Justinian, Florentinus maintains that ius gentium gives the right to defend against injury with violence.53

In the medieval and Scholastic tradition, one of the key issues was whether a Christian could partake in war and under what conditions could a war be justified. The latter issue, when war was justified, was also a prime consideration for Vitoria and other Spanish writers of the period.54 The theme of a just war was also a central preoccupation in early modern scholarship.

Gentili's eclectic collection of sources makes it quite difficult to deduce whether he is actually following any particular precedent when describing the preconditions of a just war, which he divides into divine, natural, and human reasons. For example, regarding the question whether religion may be a cause for a just war, Gentili cites the almost unanimous approval of ancient and contemporary authorities as well as the continuing practice of waging war on religious grounds, but finally takes a swift turn towards religious tolerance. Gentili claims that while it is lawful to wage war against those who disturb the public peace within the State, to wage war against people in (p. 1024) other countries whose religious practices or way of living is no imminent threat to you is in no case just.55

Gentili employs a similar line of reasoning with regards to pre-emptive strikes, following the line of argument of Roman private law which demanded a just cause for fear and allowed a suitable precaution to be taken, if such was present. Someone may not be punished for just the desire to do harm, but for Romans the imminent danger of someone both possessing the will and the means to do harm justified action, as in the case of Caesar acting against Ariovistus in Gaul when the latter had gained too much strength.56 In De armis romanis, Gentili operates within the Roman tradition of a just war, both attacking and defending the actions of the Romans by using the contradictory Roman sources that defend and criticize imperial expansion.57

Grotius proceeds in De iure belli ac pacis first to prove that war is permissible under certain circumstances under the law of nature and the law of nations. Only in book II does he treat the just causes of war: the defence of self, property, of what is owed out of contract, and what is owed out of delict. Here, Grotius follows at length the Roman argumentation, because the Romans took great pains in examining the just causes for war. Quoting Livy, he mentions that the Romans thought that their wars were victorious only as long as they were just. However, Grotius also mentions the contrary Roman tradition of criticism of the slaughter and cruelty of wars as a contradiction of the aim of society to limit murder, as formulated by Seneca. Grotius compares the just causes for war to the just causes for a lawsuit: defence, obtaining what is owed and inflicting punishment. Thus he identifies the just causes of war with the iniuriae of Roman private law. He refers also to the Roman tradition of fetial law and its practice of rerum repetitio, the demands that were made before hostilities commenced. Contrary to the claims of Gentili, Grotius would rather prohibit than allow a pre-emptive strike against a would-be attacker. Here Grotius, though he refers to the same Roman and Greek sources, lays more weight on medieval and early modern authors to produce an opposite interpretation.58

When Grotius deals later with the unjust causes for war, a special mention is reserved on one unjust cause, namely of the title of universal empire claimed by the Church. The refutation of the claim of universal empire is backed with purely theological arguments.59

(p. 1025) The use of classical precedents and lines of argumentation diminishes with later authors, as does the preoccupation with the issue of a just war. For example, Bynkershoek does not consider the question of a just war at all.60 His book on war goes straight to the point of whether a declaration of war is needed and what kind of actions are permitted in war. The answer to the first question is that it would be nice, as the Romans did issue such declarations, but not necessary, and to the second question the answer was that every act of force is lawful in war, even though the Romans preferred not to use fraud.61

However, the substantive doctrine continues without dramatic changes. Pufendorf claimed that whilst war must be the last resort, one may use force against an attacker and to force the payment of something owed.62 Wolff's concept of a just cause for war is defined as a response to a wrong done or anticipated.63

4.2.  Sanctity of Envoys

The Classical Roman jurist Pomponius clearly expressed the concept of the sanctity of ambassadors:

If someone strikes an ambassador of the enemy, he is regarded as having acted against the law of nations, because ambassadors are regarded as sacred.64

Pomponius further quotes Quintus Mucius in that whoever strikes an ambassador should be surrendered to the enemy.

In his book on embassies, Gentili notes how the sanctity of envoys was underlined by both Greeks and Romans with examples ranging from Cicero to the Roman jurists presented. However, when the general principle is applied in practice, a more nuanced picture appears and it bears less similarity to the blanket statements presented by ancient authors. Thus spurious embassies, embassies sent by brigands, and other exceptions to the rule are discussed extensively, while the whole argumentation is based on classical sources.65

(p. 1026) Grotius wrote how the sanctity of envoys was already held to be an inviolable rule of the law of nations among the Romans. Quoting the opinions of Pomponius (Dig 50.7.18) and Ulpian (Dig 48.6.7) from the Digest of Justinian, and other examples from Roman historians of the classical period and late Antiquity, Grotius holds that the ancients were nearly unanimous on the sanctity of ambassadors. The rights of ambassadors, as demonstrated by the practice of the ancients, included the right to be admitted and not to be harmed. Of the ancient examples, only in cases where there was an intense hatred or dislike of the people sending ambassadors were these rights not honoured.66

Bynkershoek acknowledged the theoretical rights of ambassadors sent by sovereign powers, but notes that such rights were commonly violated; for example, during the Dutch war of independence from Spain. While the Dutch claimed that Spain had violated the sanctity of ambassadors by murdering their envoys, Bynkershoek maintained that actually the Spanish were fully within their rights in killing them because they represented not a sovereign power but a group of rebels. Even the Romans had acted similarly.67

Wolff also stoutly defends the sanctity of ambassadors and their immunity from all kinds of violations. This sanctity is for Wolff not a result of a conscious decision or some classical precedent but rather the law of all nations derived from God himself.68

4.3.  Freedom of the High Seas

One of the important issues for early modern international law scholars was the freedom of the seas. The medieval legal doctrine, confirmed by papal bulls, was clearly against the idea of the freedom of the seas. Both Gentili and Grotius used Roman law doctrine (Institutes of Justinian 1.1) to argue that no one could claim exclusive rights or ownership to the sea, and such arguments were then used in diplomatic correspondence of Britain with Spain, and the Dutch with the Portuguese government.69

Grotius argues that ownership of the seas would require exclusive possession and such ownership is impossible because nobody can occupy the sea. Furthermore, one may not claim exclusive rights to something that is by its nature not exclusive. Grotius’ convoluted definition of non-exclusive is that if something, while serving someone, may serve someone else without limiting the use of the first one, is by nature not to be held exclusively. As Roman jurists stated, such things are by nature common.70

(p. 1027) 5.  Roman Private Law Analogies

5.1.  Pacta Sunt Servanda

Though pacta sunt servanda is one of the most famous phrases of the Roman law tradition, the expression itself is not Roman but was coined by Grotius. Despite the fact that consensual contracts of Roman law, for example, the contract of sale (emptio venditio), were free of forms, they had to meet the legal definition in order to be enforceable.71 It has been claimed that the roots of the doctrine in international law should be sought from the Canon law tradition which was also a universally accepted and applied system of law in Europe prior to the Reformation.72

On the obligations arising from promises, Grotius follows the arguments of Roman jurists in that contractual obligations should be honoured. The central component in the argument is the concept of good faith, which holds that one should keep one's promises to one another. Thus even the Roman jurist Paul maintains that the law of nations demands that if one has promised to pay then one ought to pay, there arising a moral obligation to pay.73 Never mind that Paul was actually referring to Roman substantive law, after Grotius, pacta sunt servanda was accepted as a basic rule of natural law.74

Lesaffer has claimed that the European order after 1660 and the rise of sovereign powers could only function if the rule of pacta sunt servanda was commonly accepted as the basis of the validity of treaties. They were seen as belonging to natural law and enforceable by force if necessary and to transcend the will of the States themselves.75 Pufendorf even goes so far as to claim that if someone intentionally fails to fulfil a legally binding pact that is a just cause for war.76

Bynkershoek notes that honouring contracts belongs to the realm of civil law and is dependent on good faith, but his classical references are not to Roman jurists. He claims that all agree that good faith would require that international treaties should be followed, but quotes Seneca in noting that good faith is rarely observed when it is not expedient. States are not legally bound by treaties and their observance of treaties depends on whether or not it is against the interests of the State. There is an ethical obligation, but no legally enforceable one.77

(p. 1028) Though Wolff again makes no mention of classical precedents, he holds that the good faith of treaties is sacred. Thus the inviolability of treaties and the inviolability of good faith may not be separated.78

5.2.  Terra Nullius

Roman law demanded that a thing acquired by occupation be res nullius, that is, not somebody else's property.79 Ownership of such a thing could be acquired by capturing it; for instance, it might refer to wild animals. However, should such an animal escape, or, to quote one of the issues troubling Roman jurists, the bees not return to their hive, any ownership gained by capture would be lost. The same rule would apply to booty: things captured from the enemy become our property by occupation and should we lose them we also lose ownership over them.80 The doctrine was adopted in international law under the name terra nullius,81 though in practice the adopted form was more an analogy and reinterpretation of the Roman doctrine, used with other elements as a part of imperial policies of legitimating possession.82

Although the property or other rights of peoples following different religions were not always fully recognized, to say the least, Vitoria supported the claim of Aquinas that religious matters have no bearing on the title of ownership or sovereignty. Aquinas stated that even infidels are the masters of their own property and relieving them of it would be theft.83 Thus in the case of the Spanish expansion in America, relieving the natives of their possessions could not be legal.84

Grotius notes that Roman jurists held that according to the law of nations one could only acquire property by occupation (occupatio) if it belonged to no one. In the De jure belli ac pacis, the discussion on the matter of ownerless things related mostly to wild animals, alluvial deposits forming new islands, and old islands disappearing by inundation, which was of course a matter of some importance in the Netherlands.85 However, the matter was a tad more pressing in the Mare liberum, where the (p. 1029) issue was the ownership of indigenous lands outside Europe. Following Roman law doctrine, Grotius claimed that the Portuguese could be owners by discovery only if the territories were res nullius. Since Portugal could neither claim to be the first to discover the new territories nor that they had possession of them, then their claims for exclusive rights by sovereignty were unfounded.86

Another widely circulated theory was that of original common property, which claimed that there were no ownerless things but only things in which ownership had not been individualized. For example, Pufendorf rejects Hobbes’ theory of first occupation and claims that there was an original common ownership.87 But regarding the occupation of uninhabited lands, the occupation may only extend to as much land as the occupants may use and defend. Should someone claim an entire island which would support an untold number of inhabitants based on very limited use, this claim would be invalid.88

Wolff followed the doctrine of res nullius of ownership following from occupation, but notes that it would follow from the will of the nation whether ownership was created. He mentions that the Romans had made a specific law regarding the presumption favouring occupation, but the presumption is also natural and thus in accordance to natural law. The right of occupation of uninhabited lands belongs to all nations indiscriminately. Here Wolff again refers to the Roman practice of establishing colonies.89 Typical of natural law scholarship, Wolff has a habit of mentioning that institutions originating from Roman law such as usucapio belong to natural law.90

5.3.  Uti Possidetis

The origins of the uti possidetis doctrine are to be found in the Roman law interdictum uti possidetis, which was part of ius civile.91 It was a legal remedy available to Roman citizens against disturbance in possession before and during court proceedings in which the use of force against the current possession was forbidden.92

The international law doctrine of uti possidetis was only formulated during the early 17th century, and it has been claimed that the peace treaties of Westphalia were the first to cite the doctrine in State practice. Grotius maintained that rights to immovable property taken in war require firm possession, meaning that the area is (p. 1030) fortified in such a way that a credible defence is possible.93 In one of the peace treaties of Westphalia, the Treaty of Münster between Spain and the Dutch Republic, the principle of uti possidetis was explicitly adopted for the demarcation of territories between the parties. Each side would remain effectively in possession and enjoyment of the territory they held at the time.94

Pufendorf mentions that Roman law includes property rights to things acquired from an enemy by occupancy, the res nullius rule. However, he notes that ownership of things taken from their owners in war acquires full validity only when claims to them are renounced when making peace.95

Bynkershoek confirms the doctrine of uti possidetis and follows the opinion of Paul in the Digest (Dig 41.2.3) on the extent of the possession: if a part is occupied then the whole is also occupied if that is the intention of the captor, even though Celsus (Dig disagrees.96

Again, making no references to classical material, Wolff defends the ownership and sovereignty of nations in that no nation may expel another from the territory it occupies. The law of nations protects the rights and ownership of nations as well as individuals.97

6.  Conclusion

The reception of ancient international law in the early modern era was part of a general reception of ancient, mostly Roman, law in the period. Like the reception of ancient law in general, there were various reasons for this reception: classical precedents were invoked to utilize the cultural prestige of the ancient world and guidance was sought from the technical rules of Roman private law which was perceived to have almost universal validity.

What was received was not necessarily the rules of ancient international law but the rules of Roman private law adapted to the context of international law. It is currently a matter of some debate whether there actually was a Roman system of international law in the first place. Because early modern scholarship used, for example, (p. 1031) Greek philosophical theories on natural law extensively, the actual relevance of the issue of the existence of ancient international law is slight.

This chapter examined ancient influences on early modern international law through several viewpoints: first, the transformation of the concept of ius gentium and, second, the use of the classical literary tradition as examples and context, and, finally, through six examples, the transmission of ancient Roman legal rules to the early modern doctrine of international law.

Ius gentium was originally a concept of Roman substantive law, that is, the law that was applied to peregrines or resident non-citizens. During the imperial period, the extension of citizenship made this interpretation redundant and ius gentium was increasingly understood as the law between nations. This interpretation, while it had been present, albeit marginally, even in the 1st century ad, gained prominence in late antiquity, from which it was picked up by medieval canon law scholarship. Antiquarian early modern legal scholarship returned to classical sources, but retained the view that ius gentium was law that was accepted by most nations and sources to it could be found everywhere from the Bible to the Code of Justinian. This conception was changed only by its justification by the nascent natural law scholarship, which kept the rules while dispensing with the classical references.

The use of the classical literary tradition in early modern scholarship was both extensive and eclectic. Gentili and Grotius were intimately familiar with ancient history and literature and used them as a storehouse of examples with which to illustrate and justify rules and interpretations. They clearly recognized that the expressed rules of Roman law only convey half the story and thus they quote ancient historians and philosophers to shed light on ancient practice. Classical examples were practical because they were free of the baggage that religious material had during a time of extreme controversy among the Christian churches. The examples also gave ample support for the endeavours of trading empires such as freedom of trade, imperialism and expansion. However, classical examples also gave support for the criticism of empire.

The reception of the Roman law doctrine of international law was followed through three examples: a just war, the sanctity of envoys, and the freedom of the high seas. The Roman conception of a just war was a complicated affair, but subsequent scholarship mostly adopted the right to self-defence. This right was interpreted widely where need arose, for example when justifying military expansion, and narrowly where needed, for example, denying the right to attack nations who professed a different faith. The sanctity of envoys was a clear rule of Roman law that was confirmed in several instances. It was adopted by early modern scholars, who proceeded to develop a number of exceptions to the general rule. Although the freedom of the seas was for the Romans a rule of private law, it was interpreted as a rule of sovereignty or the lack of it over the high seas, and helped to overturn the long-standing doctrine to the contrary.

(p. 1032) The reinterpretation of Roman private law analogies to the new international law doctrine was explored in three examples: the pacta sunt servanda rule, the concept of terra nullius, and the uti possidetis doctrine. Pacta sunt servanda, a principle that a contract was generally valid and enforceable, was not a rule of classical Roman law, which demanded that contracts had to meet a number of definitions to be enforceable. Grotius adopted another Roman concept, good faith, and claimed that contractual obligations should be honoured because of the presence of a moral obligation. The Roman law rule of res nullius, which stated that one gained ownership to an ownerless thing through occupation, was transformed in international law to the rule of terra nullius. The idea of terra nullius was used both to support claims to sovereignty to discovered lands during the period of European expansion and conversely to deny sovereignty because of existing indigenous title. The case of uti possidetis was a more creative adaptation because the original rule was part of the ius civile, the law applicable only to Roman citizens. During the early modern period, both Grotius in scholarship and the emerging State practice led to the interpretation that after armed conflict, concrete possession became the rule of demarcation between parties.

In most of the cases of the adoption of Roman legal rules, the trend has been that only the first generation of receivers such as Gentili and Grotius referred to the sources of the rule in ancient law. Later scholars showed a diminishing interest in the origins of the doctrine and referred mostly to the first generation of scholars.

  • Bederman, David J ‘Reception of the Classical Tradition in International Law: Grotius’ De jure belli ac pacis’ (1996) 10 Emory International Law Review 1–50.
  • Benton, Lauren and Benjamin Straumann ‘Acquiring Empire by Law: From Roman Doctrine to Early Modern European Practice’ (2010) 28 Law and History Review 1–38.
  • Lauterpacht, Hersch Private Law Sources and Analogies of International Law (Longmans, Green and Co. London 1927).
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  • Nussbaum, Arthur ‘The Significance of Roman Law in the History of International Law’(1952) 100 University of Pennsylvania Law Review 678–87.
  • Straumann, Benjamin ‘“Ancient Caesarian Lawyers” in a State of Nature: Roman Tradition and Natural Rights in Hugo Grotius's De iure praedae’ (2006) 34 Political Theory 328–50.
  • Straumann, Benjamin Hugo Grotius und die Antike (Nomos Baden-Baden 2007).
  • (p. 1033) Straumann, Benjamin and Benedict Kingsbury (eds) The Roman Foundations of the Law of Nations: Alberico Gentili and the Justice of Empire (OUP New York 2010).
  • Winkel, Laurens ‘Les origines antiques de l’appetitus societatis de Grotius’ (2000) 68 Legal History Review 393–403.
  • Winkel, Laurens ‘The Peace Treaties of Westphalia as an Instance of the Reception of Roman Law’ in Randall Lesaffer (ed) Peace Treaties and International Law in European History (CUP Cambridge 2004) 222–40.


*  The author wishes to thank Dr Benjamin Straumann and the editors for their comments.

1  The reception of Roman law from the Middle Ages onwards in Europe is the most influential example of the reception and use of ancient law. It has been studied extensively, see eg P Stein Roman Law in European History (CUP Cambridge 1999); F Wieacker A History of Private Law in Europe (OUP Oxford 1995); JQ Whitman The Legacy of Roman Law in the German Romantic Era (Princeton University Press Princeton 1990); P Koschaker Europa und das römisches Recht (4th edn Beck München 1966).

2  R Lesaffer ‘Roman Law and the Early Historiography of International Law: Ward, Wheaton, Hosack and Walker’ in T Marauhn and H Steiger (eds) Universality and Continuity in International Law (Eleven International Publishing The Hague 2011) 149–84; B Straumann Hugo Grotius und die Antike (Nomos Baden-Baden 2007); L Winkel ‘The Peace Treaties of Westphalia as an Instance of the Reception of Roman Law’ in R Lesaffer (ed) Peace Treaties and International Law in European History (CUP Cambridge 2004) 222–40; A Nussbaum ‘The Significance of Roman Law in the History of International Law’(1952) 100 University of Pennsylvania Law Review 678–87; H Lauterpacht Private Law Sources and Analogies of International Law (Longmans, Green and Co London 1927).

3  A Gentili The Wars of the Romans: A Critical Edition and Translation of De armis romanis (D Lupher trans, B Kingsbury and B Straumann eds) (OUP New York 2010); B Straumann and B Kingsbury (eds) The Roman Foundations of the Law of Nations: Alberico Gentili and the Justice of Empire (OUP New York 2010); K Tuori ‘Alberico Gentili and the Criticism of Expansion in the Roman Empire. The Invader's Remorse’ (2009) 11 Journal of the History of International Law 205–19; D Panizza ‘Political Theory and Jurisprudence in Gentili's De iure belli’ (2005) 15 International Law and Justice Working Paper; D Panizza Alberico Gentili, giurista ideologo nell’Inghilterra elisabettiana (La Garangola Padova 1981); Centro Internazionale Studi Gentiliani, Alberico Gentili e la dottrina della guerra giusta nella prospettiva di oggi. Atti del Convegno, III Giornata Gentiliana, 17 Settembre 1988 (Giuffré Milano 1991); Centro Internazionale Studi Gentiliani, Il diritto della guerra e della pace di Alberico Gentili. Atti del Convegno, IV Giornata Gentiliana, 21 Settembre 1991 (Giuffré Milano 1995); GHJ Van der Molen Alberico Gentili and the Development of International Law (Leyde 1968);Hugo Grotius und die Antike (n 2); F Mühlegger Hugo Grotius: Ein christlicher Humanist in politischer Verantwortung (Walter de Gruyter Berlin 2007); CA Stumpf The Grotian Theology of International Law (Walter de Gruyter Berlin 2006); R Tuck The Rights of War and Peace: Political Thought and the International Order from Grotius to Kant (OUP Oxford 1999); DJ Bederman ‘Reception of the Classical Tradition in International Law: Grotius’ De jure belli ac pacis’ (1996) 10 Emory International Law Review 1–50; Y Ōnuma (ed) A Normative Approach to War: Peace, War, and Justice in Hugo Grotius (Clarendon Press Oxford 1993); B Kingsbury, H Bull, and A Roberts, Hugo Grotius and International Relations (OUP New York 1990); H Vreeland Hugo Grotius (Rothman Littleton 1986); C Gellinek Hugo Grotius (Twayne Publishers Boston 1983); E Dumbould The Life and Legal Writings of Hugo Grotius (University of Oklahoma Press Norman 1969); C van Vollenhoven The Framework of De iure belli ac pacis (Noord-Hollandische Uitgeversmaatschappij Amsterdam 1931); WSM Knight The Life and Works of Hugo Grotius (Sweet and Maxwell London 1925).

4  The Rights of War and Peace (n 3); D Hüning (ed) Naturrecht und Staatstheorie bei Samuel Pufendorf (Nomos Baden-Baden 2009); A Kinji Cornelius van Bynkershoek: His Role in the History of International Law (Kluwer Hague 1998).

5  DJ Bederman International Law in Antiquity (CUP Cambridge 2001) at 16–47; A Altman ‘Tracing the Earliest Recorded Concepts of International Law—The Early Dynastic Period in Southern Mesopotamia’ (2004) 6 Journal of the History of International Law 153–72; A Altman ‘Tracing the Earliest Recorded Concepts of International Law—(2) The Old Akkadian and Ur III Periods in Mesopotamia’ (2005) 7 Journal of the History of International Law 115–36.

6  C Baldus ‘Vestigia pacis. The Roman Peace Treaty: Structure or Event?’ in Roman Law and the Early Historiography of International Law (n 2) 103–46 at 107–13; K-H Ziegler ‘The Influence of Medieval Roman Law’ in Roman Law and the Early Historiography of International Law (n 2) 147–61; N Grotkamp Völkerrecht im Prinzipat (Nomos Baden-Baden 2009); K-H Ziegler ‘Zum Völkerrecht in der römischen Antike’ in Iurisprudentia universalis: Festschrift für Theo Mayer-Maly (Böhlau Köln 2002) 933–44; International Law in Antiquity (n 5); C Phillipson The International Law and Custom of Ancient Greece and Rome (Macmillan London 1911).

7  HW Blom and LC Winkel (eds) Grotius and the Stoa (Van Gorcum Assen 2004); M von Albrecht ‘Fides und Völkerrecht von Livius bis Hugo Grotius’ in Livius, Werke und Rezeption, Festschrift für Erich Burck (CH Beck München 1983) 295–9; W Preiser ‘Die Philosophie der Stoa in ihrer Bedeutung für das moderne Völkerrecht’ (1949–50) 38 Archiv für Rechts- und Sozialphilosophie 364–70.

8  Gaius Institutiones 1.1.

9  Roman Law and the Early Historiography of International Law (n 2) 2–5. According to Lesaffer, supporting the non-continuity are inter alia International Law in Antiquity (n 5) 4–6 ‘Significance of Roman Law’ (n 2) 681; WG Grewe The Epochs of International Law (M Byers trans) (Walter de Gruyter Berlin 2000) at 9, while W Preiser Die Völkerrechtsgeschichte (Franz Steiner Wiesbaden 1964) and K-H Ziegler ‘Die römische Gründlagen des europäischen Völkerrechts’ (1972) 4 Ius Commune 1–27 are supporters of the continuity.

10  B Straumann ‘Is Modern Liberty Ancient?’ (2009) 27 Law and History Review 55–85 at 61; B Straumann ‘ “Ancient Caesarian Lawyers” in a State of Nature: Roman Tradition and Natural Rights in Hugo Grotius’ De iure praedae’ (2006) 34 Political Theory 328–50.

11  K Tuori Ancient Roman Lawyers and Modern Legal Ideals (Klostermann Frankfurt 2007).

12  The references to early modern scholarship will be in a double format. Where available, a page reference is made to the currently most commonly available English translation, the Classics of International Law series (which is also available in many online databases such as HeinOnline), after which a book, chapter, subchapter reference is made to find the corresponding passage in the original Latin.

13  ‘The Peace Treaties of Westphalia’ (n 2) 225; M Kaser Ius gentium (Böhlau Köln 1993).

14  Cicero De officiis 3.17.69: ‘itaque maiores aliud ius gentium, aliud ius civile esse voluerunt, quod civile, non idem continuo gentium, quod autem gentium, idem civile esse debet.’ See also Cicero De officiis 3.5.23: ‘natura, id est iure gentium’.

15  Cicero Tusculanae disputationes 1.13.30: ‘omni autem in re consensio omnium gentium lex

naturae putanda est’; Cicero De partitione oratoria 37.130: ‘Atque haec communia sunt naturae atque legis, sed propria legis et ea quae scripta sunt et ea quae sine litteris aut gentium iure aut maiorum more retinentur.’

16  Gaius Institutiones 1.1: ‘Omnes populi, qui legibus et moribus reguntur, partim suo proprio, partim communi omnium hominum iure utuntur: Nam quod quisque populus ipse sibi ius constituit, id ipsius proprium est vocaturque ius civile, quasi ius proprium civitatis; quod vero naturalis ratio inter omnes homines constituit, id apud omnes populos peraeque custoditur vocaturque ius gentium, quasi quo iure omnes gentes utuntur. Populus itaque Romanus partim suo proprio, partim communi omnium hominum iure utitur. Quae singula qualia sint, suis locis proponemus.’ Gaius The Institutes of Gaius (WM Gordon and OF Robinson trans) (Duckworth London 1988) at 19.

17  ‘The Peace Treaties of Westphalia’ (n 2) 225.

18  A Gellius Noctes Atticae 6.3.45: ‘quae non iure naturae aut iure gentium fieri prohibentur’.

19  Digest ‘Privatum ius tripertitum est: collectum etenim est ex naturalibus praeceptis aut gentium aut civilibus. 4. Ius gentium est, quo gentes humanae utuntur. Quod a naturali recedere facile intellegere licet, quia illud omnibus animalibus, hoc solis hominibus inter se commune sit.’ For the English translation, see The Digest of Justinian (GEM de Ste Croix trans, A Watson ed) (University of Pennsylvania Press Philadelphia 1985) at 1.

20  Digest 1.1.4.

21  Digest 1.1.6: ‘Ius civile est, quod neque in totum a naturali vel gentium recedit nec per omnia ei servit: itaque cum aliquid addimus vel detrahimus iuri communi, ius proprium, id est civile efficimus.’

22  Digest 1.1.5: ‘Ex hoc iure gentium introducta bella, discretae gentes, regna condita, dominia distincta, agris termini positi, aedificia collocata, commercium, emptiones venditiones, locationes conductiones, obligationes institutae: exceptis quibusdam quae iure civili introductae sunt.’ For the English translation see Digest of Justinian (n 19) 2.

23  Digest 50.7.18(17).

24  Isidore of Seville Etymologiae 5.6: ‘Quid sit ius gentium. Ius gentium est sedium occupatio, aedificatio, munitio, bella, captivitates, servitutes, postliminia, foedera pacis, indutiae, legatorum non violandorum religio, conubia inter alienigenas prohibita. Et inde ius gentium, quia eo iure omnes fere gentes utuntur.’ Isidore of Seville The Etymologies of Isidore of Seville (SA Barney et al trans, eds) (CUP Cambridge 2006) at 118.

25  ‘The Peace Treaties of Westphalia’ (n 2) 226.

26  Decretum Gratiani, Distinctio 1, c. 9: ‘Quid sit ius gentium. [Isidor. eod. c. 6.] Ius gentium est sedium occupatio, edificatio, munitio, bella, captiuitates, seruitutes, postliminia, federa pacis, induciae, legatorum non uiolandorum religio, conubia inter alienigenas prohibita. § 1. Hoc inde ius gentium appellatur, quia eo iure omnes fere gentes utuntur.’

27  Aquinas Summa Theologica ‘Videtur quod inconvenienter Isidorus divisionem legum humanarum ponat, sive iuris humani. Sub hoc enim iure comprehendit ius gentium, quod ideo sic nominatur, ut ipse dicit, quia eo omnes fere gentes utuntur. Sed sicut ipse dicit, ius naturale est quod est commune omnium nationum. Ergo ius gentium non continetur sub iure positivo humano, sed magis sub iure naturali. Distinguitur tamen a lege naturali, maxime ab eo quod est omnibus animalibus communis.’

28  The Roman Foundations of the Law of Nations (n 3) 1–18.

29  F de Vitoria ‘Relectio de Indis’ (JB Scott trans) in JB Scott (ed) The Spanish Origin of International Law (Clarendon Press Oxford 1934) Appendix A, at xxxvi (3.1); A Pagden and J Lawrance ‘Introduction’ in A Pagden and J Lawrance (eds) Francisco de Vitoria, Political Writings (CUP Cambridge) xv–xvi.

30  For an overview of the discussion, see R Lesaffer ‘The Grotian Tradition Revisited: Change and Continuity in the History of International Law’ (2002) 73 British Year Book of International Law 103–39 at 124.

31  A Gentili De jure belli (JC Rolfe trans) in JB Scott (ed) The Classics of International Law (Clarendon Press Oxford 1933) vol 2, at 5, 8, 10–11 (book 1, ch 1 pt 5, 10–11, 14–17); B Straumann ‘The corpus iuris as a Source of Law Between Sovereigns in Alberico Gentili's Thought’ in The Roman Foundations of the Law of Nations (n 3) 101–23.

32  H Grotius De jure belli ac pacis libri tres (FW Kelsey trans) in JB Scott (ed) The Classics of International Law (Clarendon Press Oxford 1925) vol 2, at 44 (book 1, ch 1, pt 14).

33  ‘The Peace Treaties of Westphalia’ (n 2) 237.

34  ‘Significance of Roman Law’ (n 2) 682.

35  S Pufendorf De jure naturae et gentium libri octo (CH Oldfather and WA Oldfather trans) in JB Scott (ed) The Classics of International Law (Clarendon Press Oxford 1934) at 226–7 (book 2, ch 3, 23).

36  C Wolff Jus gentium methodo scientifica pertractatum (JH Drake trans) in JB Scott (ed) The Classics of International Law (Clarendon Press Oxford 1934) vol 2, at 17–19 (paras 22–6).

37  See the contribution by M Scattola ‘Alberico Gentili (1552−1608)’ in this volume.

38  De Jure Belli (n 31) 340 (book 3, ch 11, pt 555).

39  ibid (n 31) 293 (book 3, ch 2, pt 478).

40  ibid (n 31) 42–8 (book 1, ch 10, pt 66–78).

41  The Wars of the Romans (n 3); ‘Alberico Gentili and the Criticism of Expansion’ (n 3).

42  See the contribution by P Haggenmacher ‘Hugo Grotius (1583–1645)’ in this volume.

43  De jure belli ac pacis libri tres (n 32) 383 (book 2, ch 14, pt 5–6); R Tuck Philosophy and Government (CUP Cambridge 1993) at 155.

44  ‘Is Modern Liberty Ancient?’ (n 10) 59–60, 62–3.

45  ‘Reception of the Classical Tradition in International Law’ (n 3) 4, 29.

46  See the contribution by K Haakonssen ‘Samuel Pufendorf (1632–1694)’ in this volume.

47  De jure naturae et gentium libri octo (n 35) 330–2 (book 3, ch 2, pt 1–2).

48  See the contribution by K Haakonssen ‘Christian Wolff (1679–1754)’ in this volume.

49  Jus gentium methodo scientifica pertractatum (n 36) 365 (para 707).

50  ‘Significance of Roman Law’ (n 2) 686.

51  Cicero De officiis 1.36; Cicero De re publica 2.31, 3.35; Livy 1.32.5–14. See ‘Is Modern Liberty Ancient?’ (n 10) 349 on the debates. Recently, Ando has argued that the ius fetiale was an Augustan construction, see C Ando ‘Empire and the Laws of War: A Roman Archaeology’ in The Roman Foundations of the Law of Nations (n 3) 30–52.

52  Cicero Pro Milone s 10.

53  Dig 1.1.3: ‘ut uim atque iniuriam propulsemus: nam iure hoc euenit, ut quod quisque ob tutelam corporis sui fecerit, iure fecisse existimetur, et cum inter nos cognitionem quandam natura constituit, consequens est hominem homini insidiari nefas esse.’

54  Spanish Origin of International Law (n 29) 200–1; JD Tooke The Just War in Aquinas and Grotius (SPCK London 1965); JA Fernández-Santamaria The State, War and Peace: Spanish Political Thought in the Renaissance 15161559 (CUP Cambridge 1977).

55  De Jure Belli (n 31) 40 (book 1, ch 9, p 66).

56  ibid (n 31) 62–6 (book 1, ch 14, p 99–107).

57  ‘Alberico Gentili and the Criticism of Expansion’ (n 3); The Wars of the Romans (n 3). The Spanish had employed a similar double use of the Roman tradition in the debates over the justification of colonial expansion, see D Lupher Romans in a New World (University of Michigan Press Ann Arbor 2003).

58  De jure belli ac pacis libri tres (n 32) 169–75 (book 2, ch 1, pt 1–5); ‘Is Modern Liberty Ancient?’ (n 10) 338–41; Just War in Aquinas and Grotius (n 54) 195–230.

59  De jure belli ac pacis libri tres (n 32) 553–4 (book 2, ch 22, pt 14).

60  See the contribution by K Akashi ‘Cornelius van Bynkershoek (1673−1743)’ in this volume.

61  C Bynkershoek, Quaestionum juris publici libri duo (T Frank trans) in JB Scott (ed) The Classics of International Law (Clarendon Press Oxford 1930) at 15–20 (book 1, chs 12).

62  De jure naturae et gentium libri octo (n 35) 386 (book 3, ch 4,6).

63  Jus gentium methodo scientifica pertractatum (n 36) 314–15 (para 617).

64  Dig 50.7.18(17): ‘Si quis legatum hostium pulsasset, contra ius gentium id commissum esse existimatur, quia sancti habentur legati. Et ideo si, cum legati apud nos essent gentis alicuius, bellum cum eis indictum sit, responsum est liberos eos manere: id enim iuri gentium convenit esse. Itaque eum, qui legatum pulsasset, Quintus Mucius dedi hostibus, quorum erant legati, solitus est respondere.’ For the English translation see M Crawford in Digest of Justinian (n 19). Similarly on the protection of ambassadors and the punishment of violators, Ulpian Digest 48.6.7.

65  A Gentili De legationibus libri tres (OUP New York 1924) 57–8 (book 2, ch 1).

66  De jure belli ac pacis libri tres (n 32) 438–41 (book 2, ch 18, pt 1–3)

67  Quaestionum juris publici libri duo (n 61) 156–7 (book 2, ch 3).

68  Jus gentium methodo scientifica pertractatum (n 36) 539 (para 1065).

69  See ‘Significance of Roman Law’ (n 2) 683–4.

70  H Grotius The Freedom of the Seas (OUP New York 1916) at 26–7 (ch 5); Digest 7.4.13 and 41.1.14.

71  D Johnston Roman Law in Context (CUP Cambridge 1999) at 77–84.

72  R Lesaffer ‘The Medieval Canon Law of Contract and Early Modern Treaty Law’ (2000) 2 Journal of the History of International Law 178–98.

73  De jure belli ac pacis libri tres (n 32) 329 (book 2, ch 11,1,4).

74  ‘Medieval Canon Law’ (n 72) 181–2.

75  ‘Grotian Tradition Revisited’ (n 30) 131.

76  De jure naturae et gentium libri octo (n 35) 441 (book 3, ch 7,9).

77  Quaestionum juris publici libri duo (n 61) 190–1 (book 2, ch 10).

78  Jus gentium methodo scientifica pertractatum (n 36) 282 (para 550).

79  See also the contributions by D Khan ‘Territory and Boundaries’ and by M Craven ‘Colonization and Domination’ in this volume.

80  Gaius 2.66–9, similarly Digest and ‘Item quae ex hostibus capiuntur, iure gentium statim capientium fiunt’; Digest on the rules deriving from ius gentium.

81  R Lesaffer ‘Argument from Roman Law in Current International Law: Occupation and Acquisitive Prescription’ (2005) 16 The European Journal of International Law 25–58 at 45. B Straumann criticizes in ‘Is Modern Liberty Ancient?’ (n 10) 78 Lesaffer's view of Roman law not accepting occupation as a mode of acquisition as too limited.

82  L Benton and B Straumann ‘Acquiring Empire by Law: From Roman Doctrine to Early Modern European Practice’ (2010) 28 Law and History Review 1–38.

83  ‘Relectio de Indis’ (n 29) v–ix, xi (s 1, sub-ss 4–7, 19); Summa Theologica 2.2. q 10, a 12.

84  ‘Acquiring Empire by Law’ (n 82) 22–3 et passim.

85  De jure belli ac pacis libri tres (n 32) 296–296 (book 2, ch 8, 1).

86  Freedom of the Seas (n 70) 13 (ch 2); Philosophy and Government (n 43) 176–9.

87  De jure naturae et gentium libri octo (n 35) 338 (book 3, ch 2,5).

88  ibid 386 (book 4, ch 6,3–4).

89  Jus gentium methodo (n 36) 142–3, 147–8 (paras 280, 291).

90  ibid 184–5 (paras 358).

91  See also the contributions by D Khan ‘Territory and Boundaries’ and by M Craven ‘Colonization and Domination’ in this volume.

92  Gaius 4.160: ‘uti nunc possidetis, qui minus ita possideatis, vim fieri veto’ ‘The Peace Treaties of Westphalia’ (n 2) 229–30.

93  De jure belli ac pacis libri tres (n 32) 667 (book 3, ch 6,4).

94  ‘The Peace Treaties of Westphalia’ (n 2) 230–1.

95  De jure naturae et gentium libri octo (n 35) 584–5 (book 4, ch 4,14).

96  Quaestionum juris publici libri duo (n 61) 44–5 (book 1, ch 6).

97  Jus gentium methodo scientifica pertractatum (n 36) 144 (para 282).