A war crime is a violation of the laws of war. The legal understanding of war crimes has been codified in several multilateral treaties, most notably the Geneva Conventions. More recently, the most comprehensive legal statement on war crimes was the Rome Statute of the International Criminal Court.
To be liable for a war crime, the victim must be protected under the Geneva Conventions. GC I, II, and III apply to soldiers, while GC IV applies to civilians and "unlawful combatants."
The following acts are war crimes under Rome Statute Article 8:
Prosecution for war crimes requires the existence of an armed conflict and that the perpetrator was aware of the conflict (ICC, Elements of Crimes, § 8).
In the context of command responsibility, the ICC will use an "overall control test," which requires that the defendant have "a role in organizing, coordinating or planning the military actions of the military group, in addition to financing, training and equipping the group or providing operational support to it" (ICC, Lubanga, Confirmation of Charges § 211).