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.

https://en.wikipedia.org/wiki/War_crime

It’s literally the first part of the definition.