Obama spoke after Libyan rebels, enabled by pounding air attacks by international fighter jets, grabbed back government-held territory, and on the eve of a major international conference in London dedicated to Libya's future.
"In just one month, the United States has worked with our international partners to mobilize a broad coalition, secure an international mandate to protect civilians, stop an advancing army, prevent a massacre, and establish a no-fly Zone with our allies and partners," Obama said.
Faced with a man who compared his people to "rats" and who had hanged "civilians in the streets," Obama said he had seen no choice but to act against Qadhafi's forces as they bore down on the key city of Benghazi.
He said the city could have suffered "a massacre that would have reverberated across the region and stained the conscience of the world."
"To brush aside America's responsibility as a leader and more profoundly our responsibilities to our fellow human beings would have been a betrayal of who were are," he said.
The president said US "interests and values" were at stake in the crisis, in an apparent response to those who question whether Libya represents a vital threat to the United States.
Obama did not, however, sketch an end game for the conflict, as critics warn that he may have taken sides in an intractable Middle Eastern civil war, and fear a blow to US credibility unless Qadhafi is ousted.
He made clear he wants Qadhafi gone, but warned that pursuing a military quest for regime change would backfire and exact a terrible price.
"If we tried to overthrow Qadhafi by force, our coalition would splinter."
"We would likely have to put US troops on the ground, or risk killing many civilians from the air.
"To be blunt, we went down that road in Iraq.”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."
Top international powers including Britain and France meet in London on Tuesday to assess the success of the coalition effort against Qadhafi and to consider how to oust the long-time leader with non military means.
The United States and its partners have levied punishing financial and diplomatic sanctions on the Libyan regime, seized billions of dollars in assets and are policing an arms embargo.
Obama said a "badly weakened" Qadhafi could hang on for some time, saying Libya would remain "dangerous" with him still around.
But he warned Qadhafi was on the wrong side of history. "The transition to a government that is responsive to the Libyan people will be a difficult task," Obama said, adding that it was up to the international community to join the United States in a rebuilding effort.
The president also sought to explain his criteria for committing US power abroad, as analysts seek an "Obama doctrine" as revolt sweeps the Middle East.
He said he would never shrink from using force decisively and unilaterally if the US homeland was threatened, but said America could also deploy its military in situations when it was not at risk, for instance to prevent genocide.
But he said he would seek the support of allies in such cases, telling Americans who have seen a decade of war: "The burden of action should not be America's alone."
Obama's speech was partly an answer to a storm of criticism in recent days, especially from lawmakers who argued they were not fully consulted on the Libyan operation.
Republican Senator John McCain welcomed Obama's explanation of the US intervention in Libya, but said his comment that regime change by force was not on the table was "puz AFP "Qadhafi must have been somewhat comforted by that," Obama's 2008 general election foe told CNN.
Senator John Cornyn, McCain's Republican colleague on the key Senate Committee on Armed Services, meanwhile blasted the US leader for coming before the American public too late after engaging in the Libya conflict.
"Obama failed to explain why he unilaterally took our nation to war without bothering to make the case to the US Congress," Cornyn said in a statement, adding that the administration was now "splitting the difference telling us Qadhafi must go, but refusing to do what it takes to remove him."