Speaking from the East Room of the White House, Obama said he had asked congressional leaders to postpone a vote on legislation he has been seeking to authorize the use of military force against Syria.
Acknowledging the weariness the nation feels after a decade of war in Iraq and Afghanistan, Obama said, "America is not the world's policeman."
And yet, he added, "When with modest effort and risk we can stop children from being gassed to death and thereby make our own children safer over the long run, I believe we should act. That's what makes America different. That's what makes us exceptional."
"Our ideals and principles, as well as our national security, are at stake in Syria," he declared.
The speech capped a frenzied 10-day stretch of events that began when he unexpectedly announced he was stepping back from a threatened military strike and first asking Congress to pass legislation authorizing the use of force against Assad.
With public opinion polls consistently showing widespread opposition to American military intervention, the White House has struggled mightily to generate support among lawmakers - liberal Democrats and conservative Republicans alike - who have expressed fears of involvement in yet another war in the Middle East and have questioned whether U.S. national security interests were at stake in Syria. Obama had trouble, as well, building international support for a military attack designed to degrade Assad's military.
Suddenly, though, events took another unexpected turn this week. First Russia and then Syria reacted positively to a seemingly off-hand remark from Secretary of State John Kerry indicating that the crisis could be defused if Damascus agreed to put its chemical weapons under international control.
The president said he was sending Secretary of State John Kerry to meet with Russian Foreign Minister Sergey Lavrov on Thursday, and he added, "I will continue my own discussion" with Russian President Vladimir Putin.
At the same time, he said the United States and its allies would work with Russia and China to present a resolution to the United Nations Security Council "requiring Assad to give up his chemical weapons and to ultimately destroy them under international control."
In a speech that lasted 16 minutes, Obama recounted the events of the deadly chemical weapons attack on Aug. 21 that the United States blames on Assad.
"When dictators commit atrocities, they depend upon the world to look the other way until these horrifying pictures fade from memory. But these things happened. The facts cannot be denied," he said.
The president said firmly that Assad's alleged attack was "not only a violation of international law, it's also a danger to our security."
If diplomacy now fails and the United States fails to act, he said, "the Assad regime will see no reason to stop using chemical weapons" and "other tyrants will have no reason to think twice about acquiring poison gas and using" it. Over time, he added, U.S. troops could face the threat of chemical warfare, and if fighting escapes Syria's border, "these weapons could threaten allies like Turkey, Jordan and Israel."
The president sought to deal methodically with what he said were questions asked by lawmakers and citizens who took the time to write him with their concerns about U.S. military action.
"I will not put American boots on the ground in Syria," he promised. "I will not pursue an open-ended action like Iraq or Afghanistan. I will not pursue a prolonged air campaign like Libya or Kosovo.
"This would be a targeted strike to achieve a clear objective: deterring the use of chemical weapons and degrading Assad's capabilities."