The Nuremburg Trials Definition of Aggression

An aggressor, for the purposes of this article, means that state which is first to commit any of the following actions:

1. Declaration of war upon another State.

2. Invasion by its armed forces, with or without a declaration of war, of the territory of another State.

3. Attack by its land, naval or air forces, with or without a declaration of war, on the territory, vessels or aircraft of another State.

No political, military, economic or other considerations may serve as an excuse or justification for such actions, but exercise of the right of legitimate self-defense, that is to say, resistance to an act of aggression, or action to assist a State which has been subjected to aggression, shall not constitute a war of aggression.

Source: The Avalon Project