In his first major address since ordering American airstrikes on the forces and artillery of Col. Muammar el-Qaddafi nine days ago, Mr. Obama emphasized that the United States's role in the assault would be limited, but said that America had the responsibility and the international backing to stop what he characterized as a looming genocide in the Libyan city of Benghazi.

?I refused to wait for the images of slaughter and mass graves before taking action,? Mr. Obama said.

At the same time, he said, directing American troops to forcibly remove Colonel Qaddafi from power would be a step too far, and would ?splinter? the international coalition that has moved against the Libyan government.

?To be blunt, we went down that road in Iraq,? Mr. Obama said, adding that ?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.?

Speaking in the early evening from the National Defense University in Washington, Mr. Obama said he had made good on his promise to limit American military involvement against Colonel Qaddafi?s forces ? he did not use the word ?war? to describe the action ? and he laid out a more general philosophy for the use of force.

But while Mr. Obama described a narrower role for the United States in a NATO-led operation in Libya, the American military has been carrying out an expansive and increasingly potent air campaign to compel the Libyan Army to turn against Colonel Qaddafi.

The president said he was willing to act unilaterally to defend the nation and its core interests. But in other cases, he said, when the safety of Americans is not directly threatened but where action can be justified ? in the case of genocide, humanitarian relief, regional security or economic interests ? the United States should not act alone. His statements amounted both to a rationale for multilateralism and another critique of what he has all along characterized as the excessively unilateral tendencies of the administration of George W. Bush.

?In such cases, we should not be afraid to act ? but the burden of action should not be America?s alone,? Mr. Obama said. ?Because contrary to the claims of some, American leadership is not simply a matter of going it alone and bearing all of the burden ourselves. Real leadership creates the conditions and coalitions for others to step up as well; to work with allies and partners so that they bear their share of the burden and pay their share of the costs; and to see that the principles of justice and human dignity are upheld by all.?

Mr. Obama never mentioned many of the other nations going through upheaval across the Arab world, including Yemen, Syria and Bahrain, but left little doubt that his decision to send the United States military into action in Libya was the product of a confluence of particular circumstances and opportunities.

He did not say how the intervention in Libya would end, but said the United States and its allies would seek to drive Colonel Qaddafi from power by means other than military force if necessary.

Speaking for 28 minutes, Mr. Obama addressed a number of audiences. To the American public, he tried to offer reassurance that the United States was not getting involved in another open-ended commitment in a place that few Americans had spent much time thinking about. To the democracy protesters across the Middle East, he vowed that the United States would stand by them, even as he said that ?progress will be uneven, and change will come differently in different countries,? a partial acknowledgment that complex relations between the United States and different Arab countries may make for different American responses in different countries.

?The United States will not be able to dictate the pace and scope of this change,? Mr. Obama said. But, he added, ?I believe that this movement of change cannot be turned back, and that we must stand alongside those who believe in the same core principles that have guided us through many storms: our opposition to violence directed against one?s own citizens; our support for a set of universal rights, including the freedom for people to express themselves and choose their leaders; our support for governments that are ultimately responsive to the aspirations of the people.?