Defending the first war launched on his watch, President Obama on Monday declared that the United States intervened in Libya to prevent a slaughter of civilians that would have stained the world’s conscience and “been a betrayal of who we are.” Yet he ruled out targeting Mo-ammar Gadhafi, warning that trying to oust him militarily would be a costly mistake.

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 – but offering no estimate on when the conflict might end.

He never described the U.S.-led military campaign as a “war” and gave no de-tails on its costs, but he offered an expansive case for why he believed it was in the national interest of the United States and allies to act.

In blunt terms, Obama said the U.S.-led response had stopped Gadhafi’s ad-vances and halted a sla-ughter he warned could have shaken the stability of an entire region.

“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.

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

Obama spoke to a respectful military audience at the National Defense University after, in Libya, rebel forces bore down Monday on Gadhafi with the help of airstrikes by the U.S.-led forces. The address to the nation was the president’s 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.

Amid protests and crackdowns across the Middle East and North Africa, Obama stated his case that Libya stands alone. “In this particular country, at this particular moment, we were faced with the prospect of violence on a horrific scale,” he said.

He also warned of the broader implications for the region, without naming the other countries undergoing violent upheaval.

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.”

Obama took pains to say why he chose to intervene in Libya even while acknowledging that America’s military cannot be used to stamp out every instance of repression.

“There will be times when our safety is not directly threatened, but our interests and values are,” the president said.

“Sometimes, the course of history poses challenges that threaten our common humanity and common security. … These may not be America’s problems alone, but they are important to us, and they are problems worth solving,” Obama said. “And in these circumstances, we know that the United States, as the world’s most powerful nation, will often be called upon to help.”

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.

“Broadening our military mission to include regime change would be a mistake,” Obama said. “If we tried to overthrow Gadhafi by force, our coalition would splinter. We would likely have to put U.S. troops on the ground or risk killing many civilians from the air. The dangers faced by our men and women in uniform would be far greater. So would the costs and our share of the responsibility for what comes next.”

And then he raised the issue of Iraq, 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.”

“We must always measure our interests against the need for action. But that cannot be an argument for never acting on behalf of what’s right,” the president said. “In this particular country – Libya – at this particular moment, we were faced with the prospect of violence on a horrific scale.”