in terms of US law the president and congress can pass what ever laws they need to make it "legal"
in terms of International law and the UN and binding treties and conventions
And The World's Most Authoritative Answer Is ... Among the world's foremost experts in the field of international law, the overwhelming jurisprudential consensus is that the Anglo-American invasion, conquest, and occupation of Iraq constitute three phases of one illegal war of aggression. [3]
Moreover, these experts in the international law of war deem both preventive wars and preemptive strikes to be euphemistic subcategories of outlawed wars of aggression.
And the experts' answer would hold true regardless of whether their governing legal authority was: (A) the UN Security Council Resolutions that were passed to implement the conflict-resolution provisions of the UN Charter; or (B) prior treaties and juridical holdings which have long since become general international law. [4]
Was the Iraq War legal or illegal?


