International law is the body of laws that governs interactions between international actors, including states, organizations, and individuals. It provides common conceptual language for international actors to use in their relationships. Today, international law applies to sovereign states, non-state actors, and even space. The subjects that are directly affected by international law have expanded substantially over time. In addition to regulating trade and economic matters, international law also affects human rights, ethical principles, and space law.

There are specific treaties that prohibit the use of certain weapons. For example, the Third Geneva Convention was adopted in 1929 and followed the two Hague Conventions of 1899 and 1907, and the Fourth Geneva Convention was adopted in 1949 and provides protection for civilians during war. While these treaties are important, not all nations follow them. Nevertheless, the vast majority of countries observe most of their obligations under international law. The United States, Russia, and China are notable exceptions.

Before the 18th century, international law governed relations between states through treaties. These treaties were nonbinding except in cases of honor. However, the United States Civil War changed all that. In 1863, the U.S. fought a war against the Confederate army. This law was deemed the first written recitation of the rules of war, and it led to the first prosecution for war crimes. A Confederate commandant was brought to trial for cruel treatment of prisoners in Andersonville, Georgia.