Similar concepts, such as perfidy, have existed for many centuries as customary law between civilized countries. Many of these customary laws were clarified in the Hague Conventions of 1899 and 1907. The modern concept of war crime was further developed under the auspices of the Nuremberg Trials based on the definition in the London Charter that was published on August 8, 1945. (Also see Nuremberg Principles.) Along with war crimes the charter also defined crimes against peace and crimes against humanity, which are often committed during wars and in concert with war crimes.

Article 22 of the Hague IV ("Laws of War: Laws and Customs of War on Land (Hague IV); October 18, 1907") states that "The right of belligerents to adopt means of injuring the enemy is not unlimited" and over the last century many other treaties have introduced positive laws that place constraints on belligerents (see International treaties on the laws of war). Some of the provisions, such as those in the Hague conventions, are considered to be part of customary international law, and are binding on all. Others are only binding on individuals if the belligerent power to which they belong is a party to the treaty which introduced the constraint.