I have not heard of any rule in international law that says a nation must get U.N. permission before going to war. Can you provide a link?
Legally, it would be a continuation of the Gulf War. There was no peace treaty, only a cease fire which was conditional on Iraq's abiding by the terms of Resolution 687, which it appears they have not. Resolution 1441 promised consequences without spelling out what they were, and has no provision preventing the U.S. and its allies from delivering those consequences.
The question in my mind is not the legality, but whether it is a good idea.
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