By Matt Spetalnick and Alister Bull
WASHINGTON (Reuters) - President Barack Obama told Americans on Monday that U.S. forces would not get bogged down trying to topple Muammar Gaddafi but stopped short of saying how the military campaign in Libya would end.
In a nationally televised address, Obama -- accused by many lawmakers of failing to explain the U.S. role in the Western air campaign against Gaddafi's loyalists -- said he had no choice but to act to avoid "violence on a horrific scale."
"We had a unique ability to stop that violence, an international mandate for action, a broad coalition prepared to join us, the support of Arab countries, and a plea for help from the Libyan people themselves," he said. It was his fullest defense of his strategy since air strikes began 10 days ago.
"We also had the ability to stop Gaddafi's forces in their tracks without putting American troops on the ground," Obama said.
But Obama set limits on his willingness to apply U.S. military might, making clear Washington would not act as the world's policeman "wherever repression occurs," a sign he would avoid armed entanglement in other Middle East hotspots.
Obama vowed to work with allies to hasten the day when Gaddafi leaves power but said he would not use force to topple him -- as former President George W. Bush did in ousting Saddam Hussein in the 2003 U.S.-led invasion of Iraq. Obama, elected in 2008, had opposed the Iraq war.
"To be blunt, we went down that road in Iraq," Obama told an audience of military officers at the National Defense University in Washington. "But regime change there took eight years, thousands of American and Iraqi lives, and nearly a trillion dollars. That is not something we can afford to repeat in Libya."
He spoke on the eve of a 35-nation conference in London to tackle the crisis in the North African oil-exporting country and weigh political options for ending Gaddafi's 41-year rule. NATO has agreed to take over the military lead from U.S. forces.
Obama sought to counter criticism at home that he lacked clear objectives in launching the Libya mission, but he left unanswered the question of how long U.S. forces would be involved and how they would eventually exit the conflict.
"We will deny the regime arms, cut off its supply of cash, assist the opposition, and work with other nations to hasten the day when Gaddafi leaves power," Obama said.
But he added that "it may not happen overnight" and acknowledged that Gaddafi may be able to cling to power. "Broadening our military mission to include regime change would be a mistake," he said.
Obama's challenge was to define the limited purpose and scope of the U.S. mission in Libya for Americans preoccupied with domestic economic concerns and weary of costly wars in two other Muslim countries, Iraq and Afghanistan.
But his words may not be enough to mollify Republican opponents who say he has failed to lead in recent global crises ranging from Middle East unrest to Japan's nuclear emergency.
Some Republican lawmakers complained that Obama's message still lacked clarity. "Americans still have no answer to the fundamental question: what does success in Libya look like?" said Brendan Buck, spokesman for House of Representatives Speaker John Boehner.
Obama's prime-time speech came a day after NATO agreed to assume full responsibility for military operations in Libya, ending uncertainty about who would take over the lead from U.S. forces. He said the handover would take place on Wednesday.
The alliance's decision gave a boost to Obama's effort to show Americans he was making good on his commitment to limit the U.S. military's involvement in Libya. NATO will take charge of air strikes that have targeted Gaddafi's military infrastructure as well as a no-fly zone and an arms embargo.
The White House also hopes Obama can score political points at home from gains on the battlefield by Libyan rebels emboldened by the Western air assault on Gaddafi's loyalists.
Most polls show Americans divided over the Libya mission and believe on balance the Obama administration and its allies do not have a clear goal in taking military action.
(Additional reporting by Alister Bull, Patricia Zengerle, Arshad Mohammed and Susan Cornwell; Editing by David Storey)