On that day (May 20th), the Vietnam-era law’s 60-day deadline for terminating unauthorized hostilities appeared to pass. But the White House argued that the activities of United States military forces in Libya do not amount to full-blown “hostilities” at the level necessary to involve the section of the War Powers Resolution that imposes the deadline.

... Two senior administration lawyers contended that American forces have not been in “hostilities” at least since April 7, when NATO took over leadership in maintaining a no-flight zone in Libya, and the United States took up what is mainly a supporting role — providing surveillance and refueling for allied warplanes — although unmanned drones operated by the United States periodically fire missiles as well.

They argued that United States forces are at little risk in the operation because there are no American troops on the ground and Libyan forces are unable to exchange meaningful fire with American forces. They said that there was little risk of the military mission escalating, because it is constrained by the United Nations Security Council resolution that authorized use of air power to defend civilians.

... On Wednesday, the Speaker of the House, John Boehner, Republican of Ohio... demanded a legal explanation by Friday....

Ten members of Congress — led by Rep. Dennis Kucinich, Democrat of Ohio, and Rep. Walter Jones, Republican of North Carolina — filed a lawsuit on Wednesday asking a judge to order Mr. Obama to stop the air war....

The administration had earlier argued that Mr. Obama could initiate the intervention in Libya on his own authority as commander-in-chief because it was not a “war” in the constitutional sense. It also released a memorandum by the Justice Department’s Office of Legal Counsel agreeing that he could do so unilaterally because he anticipated that its nature, scope, and duration would be limited.

Since then, the conflict in Libya has dragged on longer than expected, and the goal of the NATO allies has all but openly shifted from merely defending civilians to forcing the Libyan dictator, Col. Muammar Qaddafi, from power. But Mr. Koh and Mr. Bauer said that while regime change in Libya may be a diplomatic goal, the military mission is separate, and remains limited to protecting civilians....

Still, such previous cases typically involved peacekeeping missions in which the United States had been invited to take part, and there were only infrequent outbreaks of violence, like those in Lebanon, Somalia and Bosnia. Libya, by contrast, is an offensive mission involving sustained bombardment of a government’s forces.