President Barack Obama told the nation Monday night that he had no choice but to use military force in Libya in order to protect U.S. interests, in what was arguably his most important foreign policy speech to date.

“I am convinced that a failure to act in Libya would have carried a far greater price for America,” he said during remarks at the National Defense University in Washington, D.C.

“There will be times … when our safety is not directly threatened, but our interests and values are,” he said. “These may not be America’s problems alone, but they are important to us, and they are problems worth solving. And in these circumstances, we know that the United States … will often be called upon to help.”

Obama walked Americans through the steps he took in the weeks leading up to authorizing missile strikes in Libya. After unsuccessfully pressuring Libyan leader Moammar Gaddafi to cease violence against civilians, the United States mobilized an international coalition, secured a United Nations mandate to protect civilians and teamed up with allies to prevent civilian fatalities by applying a no-fly zone.

“We knew that if we waited one more day, Benghazi — a city nearly the size of Charlotte — could suffer a massacre that would have reverberated across the region and stained the conscience of the world,” he said. “It was not in our national interest to let that happen. I refused to let that happen.”

The president insisted that he has kept his pledge to limit U.S. involvement in scope and duration, and he announced that NATO will officially take the lead on protecting Libyan civilians Wednesday, in addition to enforcing an arms embargo and the no-fly zone.

“So for those who doubted our capacity to carry out this operation, I want to be clear: The United States of America has done what we said we would do,” he said.

Obama also pushed back on critics who have pressed for a broader military mission of regime change. “To be blunt, we went down that road in Iraq,” he said. “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.”

Secretary of State Hillary Rodham Clinton will head to London on Tuesday to consult with 30 nations on the next steps for politically pressuring Gaddafi to step down, Obama said.

Not surprisingly, Obama did not address the costs of U.S. military action or give a timeline on how long the United States will be involved in political efforts to force out Gaddafi. Members of Congress in both parties have pressed for answers on both fronts.