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."