on international law, de iure belli ac pacis, which dealt with the laws of war and peace. One important aspect of Grotius's treatment of international law is that he no longer bases it exclusively upon natural law, but also accepts that states among themselves can also create binding rules of law (ius voluntarium). Still, in the 17th and 18th centuries, the idea of natural law as a basis for international law remained influential, and were further expressed in the works of Samuel von Pufendorf and Christian Wolff. Yet, in the second half of the 18th century, a shift occurs towards positivism in international law. In addition, the idea of international law as a means for maintaining international peace is challenged due to the increasing tensions between the European great powers (France, Prussia, Great-Britain, Russia and Austria). This tension between legal norms and political imperatives is well reflected in the century's most important treatise on international law, Emer de Vattel's Du Droit des Gens (1758). At the end of the century, Immanuel Kant believed that international law as a law that can justify war does not serve the purpose of peace anymore, and therefore argues in Perpetual Peace (Zum Ewigen Frieden, 1795) and the Metaphysics of Morals (Metaphysik der Sitten, 1797) for creating a new kind of international law. After World War I, an attempt was made to establish such a new international law of peace, of which the League of Nations was considered to be one of the cornerstones, but this attempt failed. The Charter of the United Nations (1945) in fact reflects the fact that the traditional notion of state sovereignty remains the key concept in the law of nations. However, as recent research has shown, ius contra bellum (the outlawry of war) has its roots in 19th century legal and political discourse. In the historiography of international law, some German authors, most notably among them Wilhelm Grewe and Karl-Heinz Ziegler, have argued that several periods can be distinguished, such as the Spanish era (1494–1648), the French era (1648-1789/1815), the English era (1789/1815-1919) and the American era since 1919. The transitions between these eras are often marked by grand peace settlements, such as the earlier mentioned treaties of Westphalia (1645–48), the treaties of Ryswick and Utrecht (1697/1714), Vienna (1814–15), Paris (1919) and San Francisco (the UN Charter, 1945). The League of Nations Following World War I, as after the