International humanitarian law (law of war) is a field of international law regulating armed conflict between states, and more recently, between states and informal groups and individuals. See Jean Pictet, Development and Principles of International Humanitarian Law (1985). International humanitarian law governs both the legality of justifications for war (jus ad bellum, or when states can resort to war) and the legality of wartime conduct (jus in bello, or how states must behave themselves during war).
International humanitarian law should not be confused with international human rights law. International humanitarian law is one of the oldest fields of conventional international law. Core principles of international humanitarian law can be found in major international treaties such as the Geneva Conventions of 1949, and the first Geneva Convention of 1864.