War crime

War crimes are, generally, acts that violated the laws of war that applied in the jurisdiction and time of occurrence. That being said, there is abundant precedent for the trial and conviction of defendants in an ad hoc tribunal that enforced issues ex post facto, because the offense had not been considered prewar, but, such as genocide, met a consensus of qualifying as egregious conduct, perhaps under the doctrine of hostis humani generis or of just war theory.

In the present international environment, the most important definitions are in the Geneva Conventions. There are complex issues when dealing with national decisions, such as the doctrine of command responsibility stated in the in re Yamashita decision. International law has generally not stayed abreast of non-state actors accused of terrorism or genocide, democide, or "ethnic cleansing".

Post-WWII tribunals at Nuremberg and Tokyo did rule ex post facto in matters such as crimes against peace, although there was some legal background such as the Kellogg-Briand Pact and the Hague Conventions.

U.S. law

U.S. law defines war crimes principally in terms of the Geneva Conventions. The statute does not address the requirements of trial and conviction. "war crime” means any conduct—

1. defined as a grave breach in any of the international conventions signed at Geneva 12 August 1949, or any protocol to such convention to which the United States is a party;
2. prohibited by Article 23, 25, 27, or 28 of the Annex to the Hague Convention IV, Respecting the Laws and Customs of War on Land, signed 18 October 1907;
3. which constitutes a grave breach of common Article 3 (as defined in subsection (d)) when committed in the context of and in association with an armed conflict not of an international character; or
4. of a person who, in relation to an armed conflict and contrary to the provisions of the Protocol on Prohibitions or Restrictions on the Use of Mines, Booby-Traps and Other Devices as amended at Geneva on 3 May 1996 (Protocol II as amended on 3 May 1996), when the United States is a party to such Protocol, willfully kills or causes serious injury to civilians.