I posted this as a comment on a story, but it deserves to have a story of it's own:
I'm going to break this down for you step by step to show that the invasion of Iraq is not only illegal under international law, but also in violation of the constitution.
1) The UN charter defines aggression as any use of force not sanctioned by the Security Council or, as in Article 51, for immediate self defense before the Security Council can act.
2) The use of force against Iraq was not sanctioned by the Security Council, and Iraq did not invade nor posed an immediate threat to the United States.
3) Therefore, the war in Iraq is a war of aggression
4) Therefore, the war in Iraq is a violation of the UN charter
5) Article VI of the Constitution states, in part: "This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land".
6) Since the war in Iraq is in violation of the UN charter, a treaty made and therefore 'supreme Law of the Land', it is also in violation of the US Constitution.
Just for sh*ts and giggles:
The Nuremberg trials called the waging of an 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."