There is no doubt that U.S. participation in the Anglo-French-American attack on Libya is completely unconstitutional. As Glenn Greenwald has pointed out, before becoming president Barack Obama, a graduate of Harvard Law School and a former law professor, accurately described the limits of a president’s authority to initiate a war in cases where the U.S. has neither been attacked nor is in imminent danger of attack:
The President does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation.
As Commander-in-Chief, the President does have a duty to protect and defend the United States. In instances of self-defense, the President would be within his constitutional authority to act before advising Congress or seeking its consent.
The civil war in Libya is a perfect case of "a situation that does not involve stopping an actual or imminent threat to the nation." While the president is limited by the Constitution to repelling or forestalling attack, Congress can declare war for a variety of purposes beyond simple defense. But as a member of the United Nations, the U.S. must abide by the provisions of the U.N. Charter.