Under Article VI of the Constitution, our treaty agreements are part of the supreme law of the land, like the Constitution itself. Under the UN Charter, we have an agreement not to engage in crimes against peace, which are defined in the Nuremberg Principles:
A war that violates these principles violates not only international law but also American law. This is true regardless of whether Congress approves it.
The Iraq War, for example, was both a
war of aggression and a violation of UN Resolution 1441. Resolution 1441 did not authorize the use of force against Iraq without further action by the UN Security Council, and the members who signed it did so only with that understanding. So, the invasion violated our international agreements and assurances as well as the UN Charter and therefore United States law.