Every single member state of the United Nations has currently ratified the conventions, which are universally accepted as customary international law, applicable to every situation of armed conflict in the world. The Geneva Conventions are four related treaties adopted and continuously expanded from 1864 to 1949 that represent a legal basis and framework for the conduct of war under international law. In the late 20th century and early 21st century, international courts extrapolated and defined additional categories of war crimes applicable to a civil war. In 1949, the Geneva Conventions legally defined new war crimes and established that states could exercise universal jurisdiction over war criminals. In the aftermath of the Second World War, the war-crime trials of the leaders of the Axis powers established the Nuremberg principles of law, such as that international criminal law defines what is a war crime.

The formal concept of war crimes emerged from the codification of the customary international law that applied to warfare between sovereign states, such as the Lieber Code (1863) of the Union Army in the American Civil War and the Hague Conventions of 18 for international war. A war crime is a violation of the laws of war that gives rise to individual criminal responsibility for actions by combatants in action, such as intentionally killing civilians or intentionally killing prisoners of war, torture, taking hostages, unnecessarily destroying civilian property, deception by perfidy, wartime sexual violence, pillaging, and for any individual that is part of the command structure who orders any attempt to committing mass killings including genocide or ethnic cleansing, the granting of no quarter despite surrender, the conscription of children in the military and flouting the legal distinctions of proportionality and military necessity.

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