Washington -- President Barack Obama on Monday rejected criticism of his decision to commit U.S. forces to the U.N.-authorized military mission in Libya, telling the American people there were strategic and moral reasons to act.

In a nationally televised speech at the National Defense University, Obama said his administration kept its pledge that the mission would be limited in size and scope, announcing that the NATO alliance would assume full command on Wednesday.

The United States now will play "a supporting role -- including intelligence, logistical support, search-and-rescue assistance, and capabilities to jam regime communications," Obama said, noting that both the risk and cost of the operation to America "will be reduced significantly."

Some Republican and Democrats have criticized the president's policy in the war-torn North African nation. Among other things, they have questioned the purpose of the mission, as well as its cost, endgame, and consequences for the broader Arab world.

Responding directly to critics, Obama detailed some of the nuance and strategy of his Libya policy and described the complaints as presenting "false choices" on the issue.

To those who question the need for a U.S. role in the Libya mission, Obama cited several reasons for joining allies in acting on the U.N. Security Council resolution that called for no-fly zone and arms embargo in Libya and protecting civilians as necessary.

Failure to do so would have allowed Libyan leader Moammar Gadhafi to unleash his military on his own people and signaled the world that such violence would go unchallenged, Obama said.

In addition, the violence from an expected attack by Gadhafi's forces on Benghazi, the rebel stronghold of 700,000 people, would have sent thousands of refugees into neighboring Tunisia and Egypt as they deal with emerging democratic movements, the president said.

"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."

He also dismissed criticism by some conservatives that the Libyan mission fails to go far enough, particularly Gadhafi's downfall is not a specific objective.

In this case, Obama, said, making the ouster of Gadhafi a targeted outcome would have been a mistake.

"If we tried to overthrow Gadhafi by force, our coalition would splinter," he said. "We would likely have to put U.S. troops on the ground to accomplish that mission, 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."

At the same time, Obama repeated that resolving the Libya crisis requires Gadhafi's ouster, and said the coalition military mission was giving the Libyan people the opportunity to make that happen.

"It may not happen overnight, as a badly weakened Gadhafi tries desperately to hang on to power," Obama said. "But it should be clear to those around Gadhafi, and to every Libyan, that history is not on Gadhafi's side. With the time and space that we have provided for the Libyan people, they will be able to determine their own destiny, and that is how it should be. "

Looking directly into the television camera, Obama also described a policy of using military force when circumstances require action to protect vital interests and uphold core principles.

Citing times "when our safety is not directly threatened, but our interests and values are," Obama said such problems require the world's most powerful nation to play a role in helping.

"In such cases, we should not be afraid to act -- but the burden of action should not be America's alone," Obama said. "As we have in Libya, our task is instead to mobilize the international community for collective action."

In response to some conservative critics who called his response in Libya too timid and beholding to the United Nations, Obama cited the U.S. invasion of Iraq in 2003 that toppled Saddam Hussein but brought on a war now winding down more than eight years later.

"Contrary to the claims of some, American leadership is not simply a matter of going it alone and bearing all of the burden ourselves," Obama said. "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."