War of aggression
A war of aggression is a military conflict initiated by one state against another without "legal" justification, violating international law as outlined in the UN Charter.
Article 2(4) of the UN Charter prohibits the use of force against a state's territorial integrity or political independence, except in cases of self-defense (Article 51) or with UN Security Council approval. The concept was formalized after World War II, notably through the Nuremberg Trials, where initiating such a war was deemed a crime against peace.
Historically, examples include Germany's invasion of Poland in 1939. Iraq's invasion of Kuwait in 1990, the NATO invasion of Kosovo in 1999, and the United States led "coalition of willing" to violate the UN Charter in the Iraq war of 2003.