Illegal war

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 the Sicilian king Conradin 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).

In the judgment of the International Military Tribunal at Nuremberg, which followed World War II, "War is essentially an evil thing. Its consequences are not confined to the belligerent states alone, but affect the whole world. To initiate a war of aggression, therefore, 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".

The Rome Statute of the International Criminal Court refers to the crime of aggression as one of the “most serious crimes of concern to the international community”, and provides that the crime falls within the jurisdiction of the International Criminal Court (ICC). However, the Rome Statute stipulates that the ICC may not exercise its jurisdiction over the crime of aggression until such time as the states parties agree on a definition of the crime and set out the conditions under which it may be prosecuted. At the Review Conference in June 11, 2010 a total of 111 State Parties to the Court agreed by consensus to adopt a resolution accepting the definition of the crime and the conditions for the exercise of jurisdiction over this crime. The relevant amendments to the Statute, however has not been entered into force yet as of May 14, 2012.

...
Wikipedia