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.