A comparison between the Prolegomena of Chapter II of De iure praedae and the Prolegomena of De iure belli ac pacis leads to the conclusion that the ideas of Grotius on legal systematization have changed considerably between 1604 and 1625. Whereas Grotius starts in IPC with general principles with a rather unclear distinction between leges and regulae, in IBP he gives first the philosophical and theological basis of international law, intertwined by a concise set of general legal rules (IBP, Prol., 8), mostly derived from Roman law after its reception in Western Europe. The general outlines of legal systematization in the early-modern period are expounded. In the attempts of legal systematization the concept of subjective rights is essential. These subjective rights are not, as is sometimes assumed, a medieval renewal of legal technique, but can be found essentially already in classical Roman law of the first centuries AD.The institutional system is not yet visible in De iure praedae, it might be different for De iure belli ac pacis. Significant here are possibly the expressions ius ad bellum and ius in bello.
The possible precursors of the preliminary rulings according to § 234 of the Treaty of the European Union are examined with emphasis on the practice in the Imperial Chancery of the Roman Empire, on Einleitung § 47 of the Prussian Allgemeines Landrecht and on the French référé législatif.
Two texts of Francisco de Vitoria, a 16th century theologian, on the emerging concept of just war on both sides give interesting information on a less known aspect of the contemporary controversy between the Emperor Charles V and the French King François I: the position of the Duchy of Burgundy. These texts also link elegantly the theory of international law and political reality.