WASHINGTON - Vigorously defending American attacks in Libya, President Barack Obama declared Monday night that the United States intervened to prevent a slaughter of civilians that would have stained the world's conscience and "been a betrayal of who we are" as Americans. Yet he ruled out targeting Moammar Gadhafi, warning that trying to oust him militarily would be a mistake as costly as the war in Iraq.

Obama announced that NATO would take command over the entire Libya operation on Wednesday, keeping his pledge to get the U.S. out of the lead fast - but offering no estimate on when the conflict might end and no details about its costs despite demands for those answers from lawmakers.

He declined to label the U.S.-led military campaign as a "war," but made an expansive case for why he believed it was in the national interest of the United States and allies to use force.

In blunt terms, Obama said the U.S.-led response had stopped Gadhafi's advances and halted a slaughter that could have shaken the stability of an entire region. Obama cast the intervention in Libya as imperative to keep Gadhafi from killing those rebelling against him and to prevent a refugee crisis that would drive Libyans into Egypt and Tunisia, two countries emerging from their own uprisings.

"To brush aside America's responsibility as a leader and - more profoundly - our responsibilities to our fellow human beings under such circumstances would have been a betrayal of who we are," Obama said. He spoke in a televised address to the nation, delivered in front of a respectful audience of military members and diplomats.

"Some nations may be able to turn a blind eye to atrocities in other countries. The United States of America is different," Obama said. "And as president, I refused to wait for the images of slaughter and mass graves before taking action."

Obama spoke as, in Libya, rebel forces bore down Monday on Gadhafi with the help of airstrikes by the U.S.-led forces. His speech was his most aggressive attempt to answer the questions mounting from Republican critics, his own party and war-weary Americans - chiefly, why the U.S. was immersed in war in another Muslim nation.

So far, the nation is split about Obama's leadership on Libya. Across multiple polls, about half of those surveyed approve of the way Obama is handling the situation. A Pew poll out Monday found that the public does not think the United States and its allies have a clear goal in Libya - 39 percent of those polled said they do; 50 percent said they do not.

Amid protests and crackdowns across the Middle East and North Africa, Obama stated his case that Libya stands alone. Obama said the United States had a unique ability to stop the violence, an international mandate and broad coalition, and the ability to stop Gadhafi's forces without sending in American ground troops. The message to his country and the world: Libya is not a precedent for intervention anywhere else.

In essence, Obama, the Nobel Prize winner for peace, made his case for war. He spoke of justifiable intervention in times when the United States, as the world's most powerful nation, must step in to help.

"In such cases," Obama said, "we should not be afraid to act."

Reaction to the speech in Congress tended to break along partisan lines, with Republicans faulting the president for what they said was his failure to define the mission clearly.

"When our men and women in uniform are sent into harm's way, Americans and troops deserve a clear mission from our commander in chief, not a speech nine days late," said Sen. John Cornyn of Texas, a member of the Armed Services Committee and head of the Senate Republicans' political arm.

"President Obama failed to explain why he unilaterally took our nation to war without bothering to make the case to the U.S. Congress."

Obama steered away from turning this into a country-by-country dissection of the Arab revolts that are testing him at every turn. Instead, he spoke in sweeping terms to draw a connecting thread.

Citing a failure to act in Libya, he said: "The democratic impulses that are dawning across the region would be eclipsed by the darkest form of dictatorship, as repressive leaders concluded that violence is the best strategy to cling to power. The writ of the U.N. Security Council would have been shown to be little more than empty words, crippling its future credibility to uphold global peace and security."

The president also sought to address critics who have said the U.S. mission remains muddled.

Indeed, he reiterated the White House position that Gadhafi should not remain in power but the U.N. resolution that authorized power does not go that far. That gap in directives has left the White House to deal with the prospect that Gadhafi will remain indefinitely. Obama said the U.S. would try to isolate him other ways.

He said that the tasks U.S. forces were carrying out - to protect Libyan civilians and establish a no-fly zone - had international support. If the U.S. were to seek to overthrow Gadhafi by force, "our coalition would splinter," the president said.

"Broadening our military mission to include regime change would be a mistake," Obama said.

Left unclear is what happens if Gadhafi stays.

He then raised the issue of Iraq and the move to rid Iraq of Saddam Hussein, a war that deeply divided the nation and defined the presidency of George W. Bush.

"Regime change there took eight years, thousands of American and Iraqi lives and nearly a trillion dollars," Obama said. "That is not something we can afford to repeat in Libya."

Domestic politics got a nod, too, in a nation saddled in debt and embroiled over how to cut spending.

"The risk and cost of this operation - to our military and to American taxpayers - will be reduced significantly" Obama said.

The president said transferring the mission to NATO would leave the United States in a supporting role, providing intelligence, logistical support and search and rescue assistance. He said the U.S. would also use its capabilities to jam Gadhafi's means of communication.

Obama spoke before an audience at the National Defense University not far from the White House. He has tended to speak and appear more comfortable in such settings than from behind his Oval Office desk.

Associated Press writers Erica Werner, Jim Kuhnhenn, Julie Pace and Stacy Anderson contributed to this story.