The history of international law dates back to ancient times. As early as 2100 BCE, the Mesopotamian city-states of Lagash and Umma signed peace treaties. Then, from the eastern Mediterranean to East Asia, various polities entered into international pacts. But, what is the origin of international law? Why is it so important today? Here are some of the key points:

Until the mid-19th century, treaties governed relations between states. These documents were essentially unenforceable unless enforced by force and were also regarded as matters of honor. However, during the U.S. Civil War, a code of law called the Lieber Code was enacted. This code became the first written statement of war rules and led to the first war crime prosecution. In 1865, the Confederate commandant of Andersonville, Georgia, was tried for cruelty to prisoners.

International law is composed of general principles and customs of law recognized by the national legal systems. These principles are often reflected in the practice of world trade and the conduct of international hostilities. Further, international law is concerned with the environment and the commons of nations. And it can even regulate the conduct of armed conflict. But this isn’t all international law. There are many other forms of international law, such as customary international law.

The most popular form of international law is secular. It is based on the principle that international norms should be based on axiomatic truths. This philosophy was popularized by the Italian lawyer Alberico Gentili in 1598. His book, De jure belli libri tres, contains comprehensive discussions of treaties and laws of war. The book, which contains three volumes, became the basis for international law.