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Inclusion clauses and the bilateral nature of early-modern peace treaties

By: Randall Lesaffer

The resilient if undeserved position of the Peace of Westphalia as the very birth act of the sovereign states system of Europe has sometimes been shored up by scholars through the claim that Westphalia was the first pan-European, multilateral peace settlement. For this, two corroborative arguments have been brought forward. Firstly, the peace was the product of a drawn-out international conference at which numerous European powers were represented. This places the Westphalia peace conference at the head of a series of general European peace conferences that stretches over Nijmegen (1678–1679), Rijswijk (1697), Utrecht (1713), Aachen [Aix-la-Chapelle] (1748), and Vienna (1815) to Paris (1919) at the end of World War I. Secondly, some authors have pointed to the clauses in the two main treaties made in Westphalia, the peace treaties of Münster and Osnabrück of 24 October 1648, by which various European powers were ‘comprehended’. Article XVII.10 of the Osnabrück Treaty (1 CTS 119) stated:

In this peace are comprehended on behalf of the Most Serene Emperor all the confederates and adherents of his majesty, first the Catholic king, the house of Austria, from the Holy Roman Empire the electors, princes among whom also the duke of Savoy, and other estates, herein comprehended the free and immediate nobility of the Empire and the Hanseatic cities; further the king of England, the king and kingdoms of Denmark and Norway with their annexed provinces such as also the duchy of Schleswig, the king of Poland, the duke of Lorraine and all the princes and republics of Italy; and the confederated estates of the Netherlands and of Switzerland and the Grisons; also the prince of Transylvania (my translation).

Article XVII.11 listed the allies of the Emperor’s counterpart, the Queen of Sweden. Paragraph 119 of the Munster Peace Treaty (1 CTS 271) also provided for the inclusion of the allies and friends of the treaty parties, the Emperor and the King of France. Only a few were mentioned by name, chiefly the Venetian Republic, which had been acting as mediator to the peace negotiations. The paragraph, however, stipulated that the signatory parties to the treaty would nominate the powers they wanted comprehended within six months. In total, the list of comprehended parties to the Westphalia Peace Treaty runs long and, as some scholars have pointed out, includes most of the European great and middle powers, with few exceptions.

However often these arguments have been repeated in the past to support the alleged constitutional role of the Westphalia Peace to the legal order of Europe, they are misguided. Although indeed quite a number of the great and middle powers of Europe were represented at the Westphalia peace conference, the conference did not end in a pan-European peace. The two greatest powers, Spain and France, which had faced one another in open war since 1635, failed to make their peace at Westphalia and would wage war until the Peace of the Pyrenees of 7 November 1659 (5 CTS 325). Moreover, as was the case with almost all multilateral peace conferences until the early 19th century, the conference did not produce a general, or even multilateral, peace treaty. The three major peace treaties signed at the conference were bilateral peace treaties. Next to the peace treaties between the Empire and France, signed at Münster on 24 October 1648, and between the Empire and Sweden, signed at Osnabrück on the same day, there was the Dutch-Spanish Treaty of Münster of 30 January 1648 (1 CTS 1).

The comprehension, or inclusion – to use the more frequent term – of allies in the Münster and Osnabrück Peace Treaties of October 1648 does not alter the bilateral nature of the treaties. Firstly, inclusion clauses were very common in early-modern European peace treaties. They first appeared in the Late Middle Ages to become an almost standard feature of peace treaties from the late 15th century onwards. In the latter third of the 17th century they started to become less frequent and elaborate, gradually disappearing over the course of the 18th century. At that time, they were increasingly replaced by guarantee clauses, whereby third parties were invited to guarantee the implementation of the treaty. In many peace treaties, the majority of the leading powers was included. If the inclusion clauses of Westphalia made that peace a pan-European one, then almost all peace treaties from the century and a half before Westphalia were pan-European as well. So if Westphalia was a general peace as has been claimed, it was by no means the first. But it was not.

Secondly, contrary to the position that has been argued by the Dutch international lawyer Johan Hendrik Willem Verzijl (Verzijl 1973, p. 203), these inclusion clauses did not amount to a full accession to the treaties of which they were a part. They did not imply that the included parties became full parties to the treaties, nor that all the treaty clauses became binding upon them. For many of the more detailed provisions of the peace treaties, this would be irrelevant or even impossible. What then were the legal consequences of inclusion in a peace treaty, if not full accession?

Contemporary scholars of the law of nations, with few exceptions, have had little to say on the subject of inclusion. The legal implications of inclusion thus need to be established on the basis of treaty practice itself. A wide-ranging research about concrete invocations of the status of included power through a survey of diplomatic sources would be needed to gain a deeper insight, but remains to be done.

From a general analysis of early-modern treaty practice, it can be concluded that the primary purpose of including one’s allies and friends in a peace treaty was political. It first and foremost served to provide reassurance to friendly powers who had shown some favour in the war to one of the sides in the war or who might find themselves open to accusations of not having been strictly neutral. It secured the guarantee that the peace would not harm them by laying them open to retribution by the nominating party’s former enemy. According to Christian Wolff (1679–1754) and Emer de Vattel (1714–1767), inclusions were most relevant to veritable allies of the belligerents, who under their treaty obligations had given active support to one belligerent against another. This referred to the status of auxiliary power, whereby a state could go as far as to give subsidies and arms or even have troops fight at the side of its ally, while remaining officially at peace with its ally’s enemy. Such auxiliaries did not need to conclude a peace treaty with their ally’s enemy, nor did they need to accede to one. For them, it sufficed to be included. This particular use of inclusion clauses thus stemmed from the distinction the early-modern laws of war made between open war and auxiliary status, or active, even direct, military support to an ally without formally disrupting peace. The rise of a stricter neutrality over the 18th century may thus be another reason why inclusion clauses lost ground at that time.

The exact legal implications of inclusion varied somewhat from treaty to treaty. In some cases, the inclusion clauses themselves expressly listed specific consequences. In most cases, however, they did not. Generally speaking, the inclusion clauses established or confirmed that a state of amicitia (friendship) would exist between the principal treaty parties and the included powers. Amicitia is an old conception of international relations and law that has its roots in classical and even pre-classical antiquity. It is a more positive qualification of peace, beyond the absence of war. It implies a promise by the parties neither to harm one another or one another’s subjects, nor to allow such harm to occur. This in turn often included a promise not to discriminate against one another’s subjects but to give them the same protection of the law and access to the justice system in one’s lands as one’s own subjects would receive.

For peace treaties, inclusion meant that the nominated and included ally and the former enemy of the nominating party would desist from all hostile or otherwise harmful action against one another. For active auxiliaries which for all practical purposes had participated in the war without openly declaring it, this meant putting an end to actual hostilities and thus came very close to peace. For more passive or truly neutral powers, it meant little difference to the pre-existing state. But in neither case did inclusion by itself amount to accession or change the bilateral nature of the peace treaty.


Randall Lesaffer, ‘The Westphalia Peace Treaties and the Development of the Tradition of Great European Peace Treaties Prior to 1648’, Grotiana, N.S. 18 (1997) 71–95.

Heinhard Steiger, ‘Der Westfälische Frieden – Grundgesetz für Europa?’ in Heinz Duchhardt (ed.), Der Westfälische Friede. Diplomatie, politische Zäsur, kulturelles Umfeld, Rezeptionsgeschichte (Munich: Oldenbourg, 1998), 33–80.

Emer de Vattel, Le droit des gens, ou principes de la loi naturelle (1758, Classics of International Law, Washington: Carnegie Institute of Washington, 1916, 3 vols.).

Johan Hendrik Willem Verzijl, International Law in Historical Perspective, vol. VI (Leiden: A.W. Sijthoff, 1973).

Christian Wolff, Jus gentium methodo scientifica pertractatum (1749, 1764 edition published here, Classics of International Law, Oxford/London: Clarendon Press/Humphrey Milford, 1934, 2 vols.).