As interpreted by Dutch jurist and humanist Hugo Grotius (1583–1645), natural law prescribes the rules of conduct among nations, resulting in international laws. His 1625 work, titled Concerning the Law of War and Peace, is considered the definitive text on international law, asserting the sovereignty and legal equality of all states of the world. But the notion also had its detractors, English philosopher Thomas Hobbes (1588–1679) among them. Hobbes insisted that since international law is not enforced by any legal body above the nations themselves, it is not legitimate.

Since the seventeenth century, however, international law has evolved to become more than just theory. During the 1800s and early 1900s, the Geneva Conventions (1864, 1906, 1929, 1949) and the Hague Conferences (1899, 1907) set forth the rules of war. Today, treaties (between two or among many countries), customary laws, legal writings, and conventions all influence international law, which is also referred to as “the law of nations.” Further, it is enforced by the International Court of Justice (a United Nations body) as well as by world opinion, international sanctions, and the intervention of the United Nations (apart from the International Court of Justice).