What ideas about war did Hugo Grotius explore in On the Law of War and Peace?

In his 1625 masterpiece, De Jure Belli ac Pacis (On the Law of War and Peace), Hugo Grotius sought to minimize bloodshed by establishing a framework to regulate war and violence between independent powers. Influenced by the brutality of the Thirty Years’ War, he argued that a common law exists that remains valid both during and in relation to conflict.12

Grotius placed natural law at the center of his theory, arguing it should be independent of religion to apply universally. However, he employed a multilayered approach, incorporating Christian principles—such as the "law of love"—and utilitarian arguments when secular norms alone were insufficient to restrain rulers.1

Regarding the legitimacy of conflict, Grotius maintained that only wars with just causes should be permitted, such as defense or the restitution of property. He viewed war as a necessary, albeit regrettable, means of resolving disputes in the absence of an international judge, and he developed a theory of crime and punishment to justify certain wars as a response to wrongdoing.1