The United Nations Treaty specifically allows for the situation where states are acting in their own defence in face of an invasion or similar hostile acts, and conditions for that defence are prescribed. The scheduled war doesn't fall within those conditions, and so far as I know, nobody has claimed that it would.

(In the unlikely - because suicidal - event that Iraq were to launch a war, in response to bombing raids, I think it is likely that this would in fact fall within the definition of legitimate self-defence.)

Just saying you believe you are acting in self-defence isn't sufficient. After all Nazi Germany claimed that when it attacked Poland.

Here's a quote which sums up the legal situation as I understand it:

"Since the signing of the UN Charter in June 1945, the only body with the authority to initiate military action is the United Nations Security Council, except in the case of self-defence when an armed attack has actually occurred against a sovereign state. Even then, the exception of self-defence, like all exceptions, is to be strictly construed.

All signatories are bound by Article 2.4 of the Charter which says that 'all members shall refrain in their international relations from the threat or use of force …' Today, in the light of the UN Charter, especially Articles 2 and 51, it is plain that the only circumstance under which a sovereign state might invoke the authority to go to war is when an armed attack occurs; even in self-defence, it may do so only 'until the Security Council has taken measures necessary to maintain international peace and security' (Article 51)."

That comes from the Pax Christi site"./a>, which is obviously a source which has a more rigorous view about what might constitute a legal or a just war than some other sites. But I'd be surprised to find an authoritative site that would disagree essentially with that interpretation of the legal position.