International humanitarian law (IHL), also referred to as the laws of armed conflict, is the law that regulates the conduct of war (jus in bello). It is a branch of international law that seeks to limit the effects of armed conflict by protecting persons who are not participants in hostilities and by restricting and regulating the means and methods of warfare available to combatants.

International humanitarian law is inspired by considerations of humanity and the mitigation of human suffering. It comprises a set of rules, which is established by treaty or custom and that seeks to protect persons and property/objects that are or may be affected by armed conflict, and it limits the rights of parties to a conflict to use methods and means of warfare of their choice. Sources of international law include international agreements (the Geneva Conventions), customary international law, general principles of nations, and case law. It defines the conduct and responsibilities of belligerent nations, neutral nations, and individuals engaged in warfare, in relation to each other and to protected persons, usually meaning non-combatants. It is designed to balance humanitarian concerns and military necessity, and subjects warfare to the rule of law by limiting its destructive effect and alleviating human suffering. Serious violations of international humanitarian law are called war crimes.

While IHL (jus in bello) concerns the rules and principles governing the conduct of warfare once armed conflict has begun, jus ad bellum pertains to the justification for resorting to war and includes the crime of aggression. Together the jus in bello and jus ad bellum comprise the two strands of the laws of war governing all aspects of international armed conflicts. The law is mandatory for nations bound by the appropriate treaties. There are also other customary unwritten rules of war, many of which were explored at the Nuremberg trials. IHL operates on a strict division between rules applicable in international armed conflict and internal armed conflict.

Since its inception, IHL has faced criticism for the fact that the foreseeable killing of large numbers of citizens can be considered compliant with IHL, and its creation largely by Western powers in service of their own interests. There is academic debate whether IHL, which is formally constructed as a system that prohibits certain acts, can also facilitate violence against civilians when belligerents argue that their attacks are compliant with IHL.

Modern international humanitarian law is made up of two historical streams:

The two streams take their names from a number of international conferences that took place in those two cities and which produced treaties relating to war and conflict, in particular the Hague Conventions of 1899 and 1907, and the Geneva Conventions, the first of which was drawn up in 1863. Both deal with jus in bello, which deals with the question of whether certain practices are acceptable during armed conflict.

The Law of The Hague, or the laws of war proper, "determines the rights and duties of belligerents in the conduct of operations and limits the choice of means in doing harm". In particular, it concerns itself with:

Systematic attempts to limit the savagery of warfare only began to develop in the 19th century. Such concerns were able to build on the changing view of warfare by states influenced by the Age of Enlightenment. The purpose of warfare was to overcome the enemy state, which could be done by disabling the enemy combatants. Thus, "the distinction between combatants and civilians, the requirement that wounded and captured enemy combatants must be treated humanely, and that quarter must be given, some of the pillars of modern humanitarian law, all follow from this principle".