President Obama, defending his decision to send U.S. forces to Libya while outlining in unprecedented detail his foreign policy philosophy, said Monday that the United States acted to "prevent a massacre" and that to do otherwise would have been "a betrayal of who we are." 

The president, in a nationally televised address, stressed that he would not commit the U.S. military to toppling Libyan leader Muammar al-Qaddafi's regime, warning that such a goal of regime change would fracture the coalition and require ground forces. He described that mission, rather, as one for the Libyan people, claiming the United States and its allies have "stopped Qaddafi's deadly advance" and will "keep the pressure" on his regime as the rebels continue fighting. 

"They will be able to determine their own destiny, and that is how it should be," Obama said. 

The president's speech, delivered nine days after he authorized military action in support of a U.N.-approved no-fly zone, was meant to answer mounting questions about the direction and purpose of the mission. The president once again described U.S. involvement as "limited" and said NATO would assume full control of the operation Wednesday -- though the president did not provide details about the conditions under which U.S. forces could leave the region. That omission prompted several complaints from lawmakers Monday night. 

But Obama also used the occasion to outline what could be called the Obama doctrine. Under that world view, the United States may intervene abroad to prevent humanitarian crises, provided international partners are involved. 

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"There will be times, though, when our safety is not directly threatened, but our interests and our values are," Obama said. "In such cases, we should not be afraid to act -- but the burden of action should not be America's alone." 

The president argued that the crisis in Libya represented such a scenario. Faced with the possibility of massive bloodshed in Libya at the hands of Qaddafi -- whom he described as a "tyrant" and blamed for the deaths of Americans in the 1988 Lockerbie bombing -- Obama said U.S. interests and values were at stake, and the country had "a responsibility to act." He said "countless lives have been saved" as a result, and suggested that a massacre in Libya could have interrupted the momentum toward political reform elsewhere in the region. 

"While I will never minimize the costs involved in military action, I am convinced that a failure to act in Libya would have carried a far greater price for America," Obama said. 

The president, unlike his predecessors, chose to deliver an address about U.S. military action not from the Oval Office, but before a military audience at the National Defense University in Washington, D.C. He also spoke more than a week after U.S. forces started launching missiles into Libya -- whereas his predecessors typically addressed the public from the very start of military involvement overseas. 

That delay compounded the criticism the president has faced from Congress. Some lawmakers say he should have first sought permission on Capitol Hill before committing U.S. forces; others complain that he has not been clear about whether the United States would be willing to end the military mission with Qaddafi still in power. 

Those complaints did not abate following the speech. Sen. John Cornyn, R-Texas, complaining that the speech came "nine days late," said afterward that the president "failed to explain why he unilaterally took our nation to war without bothering to make the case to the U.S. Congress." He also accused the president of "splitting the difference" by continuing to call for Qaddafi's removal without explaining what it would take to remove him. 

"Nine days into this military intervention, Americans still have no answer to the fundamental question -- what does success in Libya look like?" said Mike Steel, spokesman for House Speaker John Boehner. 

Obama, though, described as a "false choice" the debate in Washington between those who believe the United States should not have intervened and those calling for an all-out campaign for regime change. 

"To be blunt, we went down that road in Iraq," Obama said of the latter argument. "That is not something we can afford to repeat in Libya." 

He said the U.S. role going forward would consist of intelligence operations, search-and-rescue missions, logistical support and help jamming Qaddafi's communications. 

Other lawmakers continued to applaud the president for his handling of the Libyan crisis. House Democratic Leader Nancy Pelosi, D-Calif., said the military intervened to "stave off a humanitarian crisis." 

The president spoke Monday as rebel forces continued to clash with those loyal to Qaddafi's regime. The protection from western allies has undoubtedly helped the rebels recover lost ground, but the outcome was still unclear. Rebels were advancing toward Qaddafi's hometown of Sirte on Monday, but a top U.S. army official said the opposition is not organized and "not a very robust organization." 

Defense Secretary Robert Gates said candidly Sunday that nobody knows how long the U.S. military could be involved in the conflict. 

Before his speech, Obama spoke Monday by videoconference with French President Nicolas Sarkozy, German Chancellor Angela Merkel and British Prime Minister David Cameron about the transition to NATO command.