Article 51
Nothing in the present Charter shall impair the inherent right of individual or collective self-defense if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defense shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.
Then there's the violations of US law the run-up to the war and the violations of the Geneva Conventions during the prosecution of the war.
******************
Preemptive War and International Law
Steven C. Welsh, Esq., CDI Research Analyst
Dec. 5, 2003
The Bush National Security Strategy has prompted continuing discussion over the legal and policy implications of preemptive military action and its impact on the future of the global security system. This backgrounder examines some of the international legal standards and related policy considerations forming the context of that debate.
Link:
http://www.cdi.org/news/law/preemptive-war.cfm