Early Theory of International Law
Dutch diplomat and scholar Hugo Grotius is commonly regarded as one of the founders of international law theory. He understood natural law as universal rules of conduct that supersede the man-made laws commonly known as positive law. Grotius believed states should not impose their ideologies upon each other and should not interfere in each other’s domestic affairs.
In Chapter 1 of De Jure Belli Ac Pacis (On the Law of War and Peace), Grotius lays out the conditions under which war is called for.