In the legal memo provided by the President's Office of Legal Counsel, the Administration argues that the President had the authority to attack Libya absent Congressional authorization because he determined it was in the national interest and because the U.S. is engaged in limited military operations that do not constitute a war.
The war in Libya is not in our national interest. The claim that the U.S. had to act in Libya in order to maintain stability in the region -- "a vital U.S. interest" -- runs contrary to the history of U.S. military intervention in the region. As evidenced by U.S. intervention in Afghanistan, Iraq and drone bombing campaigns in Pakistan, rather than maintain stability, U.S. military action in the region has unfortunately served to further instability. Occupations fuel insurgencies and close a circle of never-ending violence. Additionally, the doctrine that the U.S. has a responsibility to act militarily, without prior authorization from Congress, in the event of a threat to any of our friends in the world puts us on a path to permanent war and has no legal basis in the Constitution or the War Powers Act.
The Obama Administration has prosecuted a war that is "not a war." The assertion that U.S. military actions in Libya do not constitute war belies the significant use of military force in Libya. The Administration's own Secretary of Defense, while testifying before Congress last month, admitted that enforcing a no-fly zone in Libya was an act of war: "A no-fly zone begins with an attack on Libya to destroy the air defenses."  The United States, thus far, has spent well over $550 million on the war in Libya, using at least 112 long-range Tomahawk cruise missiles, estimated to cost up to $1.5 million each, in the first day alone. The U.S. also used Joint Direct Attack Munitions -- 2,000 pound bombs -- to bomb Libya. The characterization of the use of force in Libya solely as a humanitarian intervention cannot hide the reality of what war is. The attempt to assert that this is not a war does violence to cognition and violence to the English language. It is positively Orwellian.
The Administration also claims that authority to use U.S. military force abroad was provided by United Nations Security Council Resolution (UNSCR) 1973, which authorized member states to "take all necessary measures" to protect Libyan civilians and to enforce a no-fly zone. The Constitution does not provide an exception for the President to unilaterally decide to use military force abroad if an international body, such as the United Nations, provides him with one. It is unequivocally clear, in Article 1, Section 8, that the power to authorize the use of military force or to declare war lies solely with Congress.
The law provides the President with the authority to use military force absent prior Congressional authorization only to repel sudden or imminent attack. There was no threat of sudden or imminent attack to the United States from Libya. President Obama himself recognized the constitutional limitations imposed on any U.S. President when, in an October 2008 interview, he stated that "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." 
In the sophistry of the Office of Legal Counsel's memo, the Obama administration fails to justify what cannot be justified. While the President has argued that the credibility of the United Nations (U.N.) was at stake if members of the Security Council did not act, it is actually the credibility of his administration and of our own democracy that is at stake. Preserving the credibility of the U.N. has never been a reason to go to war. A plain reading of the U.S. Constitution explicitly places war powers in the hands of Congress.
In this flimsy attempt to justify military action in Libya, it appears as though the administration is taking scissors and scotch tape to the Constitution, cutting out sections they do not like, and replacing them with legal theory that is reminiscent of the now discredited theories (of a former administration) which were used to justify torture.