1. UN Charter < US Constitution in terms of legitimizing United States political acts.
2. Saddam Hussein was in violation of UN Security Council Resolution 660, UN Security Council Resolution 661, UN Security Council Resolution 678, UN Security Council Resolution 686, UN Security Council Resolution 687, UN Security Council Resolution 688, UN Security Council Resolution 707, UN Security Council Resolution 715, UN Security Council Resolution 986, and UN Security Council Resolution 1284.
3. Ergo, UN Security Council 1441 gave Saddam Hussein "a final opportunity" to comply. He refused, we enforced the "serious consequences" it promised him for that refusal.
If the US needed it (which it didn't), we did indeed have the added justification of enforcing the UN Security Resolutions.