A war of aggression, sometimes also war of conquest, is a military conflict waged without the justification of self-defense, usually for territorial gain and subjugation. The phrase is distinctly modern and diametrically opposed to the prior legal international standard of "might makes right", under the medieval and pre-historic beliefs of right of conquest. Since the Korean War of the early 1950s, waging such a war of aggression is a crime under the customary international law. Possibly the first trial for waging aggressive war is that of Conradin von Hohenstaufen in 1268.

Wars without international legality (e.g. not out of self-defense nor sanctioned by the United Nations Security Council) can be considered wars of aggression; however, this alone usually does not constitute the definition of a war of aggression; certain wars may be unlawful but not aggressive (a war to settle a boundary dispute where the initiator has a reasonable claim, and limited aims, is one example).

The International Military Tribunal at Nuremberg, which followed World War II, called the waging of aggressive war "essentially an evil thing...to initiate a war of aggression...is not only an international crime; it is the supreme international crime, differing only from other war crimes in that it contains within itself the accumulated evil of the whole." Article 39 of the United Nations Charter provides that the UN Security Council shall determine the existence of any act of aggression and "shall make recommendations, or decide what measures shall be taken in accordance with Articles 41 and 42, to maintain or restore international peace and security".