here's the key excerpt from the march 17 UN security council press release, laying down the law on military action in libya:

Adopting resolution 1973 (2011) by a vote of 10 in favour to none against, with 5 abstentions (Brazil, China, Germany, India, Russian Federation), the Council authorized Member States, acting nationally or through regional organizations or arrangements, to take all necessary measures to protect civilians under threat of attack in the country, including Benghazi, while excluding a foreign occupation force of any form on any part of Libyan territory....

if you wonder why i call it "law," recall that the UN charter is a treaty signed by the president and ratified by the senate, and the constitution says treaties are part of the supreme law of the land:

The U.S. Senate, by a vote of 89 to 2, gave its consent to the ratification of the UN Charter on July 28, 1945.
http://www.sourcewatch.org/index.php?title=United_Nations

Article VI
This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land....
http://www.usconstitution.net/const.html

and that's why obama didn't need congressional approval, which he couldn't have gotten in time to stop a slaughter, because we all know damn well the gops would've delayed it.

(i'm not arguing morality here, just legality.)