A crime of aggression or crime against peace is the planning, initiation, or execution of a large-scale and serious act of aggression using state military force. The definition and scope of the crime is controversial. The Rome Statute contains an exhaustive list of acts of aggression that can give rise to individual criminal responsibility, which include invasion, military occupation, annexation by the use of force, bombardment, and military blockade of ports. Aggression is generally a leadership crime that can only be committed by those with the power to shape a state's policy of aggression, rather than those who carry it out.

The philosophical basis for the wrongness of aggression is found in just war theory, according to which a war fought for territorial aggrandizement is unjust. In the wake of the German invasion of the Soviet Union during World War II, Soviet jurist Aron Trainin made the first successful proposal to criminalize aggression. The Charter of the International Military Tribunal provided criminal liability for waging aggressive war, which was the main focus of the Nuremberg trial. Other participants in World War II were tried for aggression in Finland, Poland, China, the subsequent Nuremberg trials, and the Tokyo trial. No one has been prosecuted for aggression either before or since the 1940s.

It is generally accepted that the crime of aggression exists in international customary law. The definitions and the conditions for the exercise of jurisdiction over this crime by the International Criminal Court were adopted in 2010 at the Kampala Review Conference by the states parties to the court. Aggression is criminalized according to the statute law of some countries, and can be prosecuted under universal jurisdiction.

Aggression is one of the core crimes in international criminal law, alongside genocide, crimes against humanity, and war crimes. In 1946, the International Military Tribunal ruled that aggression was "the supreme international crime" because "it contains within itself the accumulated evil of the whole". The standard view is that aggression is a crime against the state that is attacked, but it can also be considered a crime against individuals who are killed or harmed as a result of war.

The Rome Statute of the International Criminal Court is the treaty that established the International Criminal Court (ICC). It was adopted at a diplomatic conference in Rome, Italy on 17 July 1998 and it entered into force on 1 July 2002. As of November 2019, 123 states are party to the statute. Among other things, the statute establishes the court's functions, jurisdiction and structure.

The Rome Statute established four core international crimes: genocide, crimes against humanity, war crimes, and the crime of aggression. Those crimes "shall not be subject to any statute of limitations". Under the Rome Statute, the ICC can only investigate and prosecute the four core international crimes in situations where states are "unable" or "unwilling" to do so themselves; the jurisdiction of the court is complementary to jurisdictions of domestic courts. The court has jurisdiction over crimes only if they are committed in the territory of a state party or if they are committed by a national of a state party; an exception to this rule is that the ICC may also have jurisdiction over crimes if its jurisdiction is authorized by the United Nations Security Council.