Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack
occurs against a Member of the United Nations (Article 51 of the UN Charter) is the relevant bit here.

Very close to all analysists agree that a US war against Iraq cannot be founded on this article (the opinion was much more split, with good reasons, in the Afghanistan case). That's why most of the US argumentation does not mention article 51 but resolution 1441 which was very carefully worded to be ambiguous.

Wolfgang