Abstract: In theory, law of war is generally divided into two parts: right to war (ius ad bellum) and law of war (ius in bello). The reason for war constitutes the basic problem of the theory of right to war, which endows the right to war with normalization. The basic reason of war originates from the ancient appeal for “justice”. However, the ways of realization and political significance of justice of war have undergone fundamental changes in early modern Europe. The rapid development of capitalism and the strong rise of commercial groups play an important role in this process, which has been expressed systematically and clearly by thinkers such as Vitoria and Grotius. Commerce is not so gentle and harmless as the later enlightener described. Both the ancient law of nations (ius gentium) and the new natural law have become the intermediary force driving the war with commercial reasons.