No American boots on the ground
"I know that after the terrible toll of Iraq and Afghanistan, the idea of any military action -- no matter how limited -- is not going to be popular," he said, later declaring: "I will not put American boots on the ground in Syria. 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."
At the same time, he rejected criticism that such a limited military response would prove meaningless, saying "the United States military doesn't do pin pricks."
"Even a limited strike will send a message to Assad that no other nation can deliver," Obama said. "I don't think we should remove another dictator with force. We learned from Iraq that doing so makes us responsible for all that comes next. But a targeted strike can makes Assad -- or any other dictator -- think twice before using chemical weapons."
He described the U.S. role in the world as "doing more than forging international agreements; it has meant enforcing them."
"The burdens of leadership are often heavy, but the world's a better place because we have borne them," Obama said before making a direct appeal to both sides of the political spectrum.
"To my friends on the right, I ask you to reconcile your commitment to America's military might with the failure to act when a cause is so plainly just," he said. "To my friends on the left, I ask you to reconcile your belief in freedom and dignity for all people with those images of children writhing in pain and going still on a cold hospital floor, for sometimes resolutions and statements of condemnation are simply not enough."
He concluded by challenging "every member of Congress and those of you watching at home tonight to view those videos of the attack, and then ask what kind of world will we live in if the United States of America sees a dictator brazenly violate international law with poison gas and we choose to look the other way?"
McCain, Graham react
McCain and fellow GOP Sen. Lindsey Graham of South Carolina responded by saying Obama failed to speak forcefully enough "about the need to increase our military assistance to moderate opposition forces in Syria, such as the Free Syrian Army."
They called for the United States and its allies on the U.N. Security Council to "immediately" offer a resolution "that lays out what steps Syria would have to take to give up its chemical weapons, including making a full and accurate declaration of all of its chemical weapons and granting international monitors unfettered access to all sites in Syria that possess these weapons."
"This resolution would have to threaten serious consequences if the Assad regime does not comply," they said, calling for an up-or-down vote by the Security Council, where Syrian allies Russia and China have so far blocked any U.N. action against Syria.
Rep. Elijah Cummings, a Maryland Democrat and staunch Obama supporter who is undecided on Syria, praised him for "a very good job" but said his position on the issue remained unchanged.
"It was not a wasted speech," said Cummings, noting his constituents are tired of war. "I thought he made a great moral argument."
Obama's reluctance on Syria
For two years, Obama resisted calls by conservative hawks such McCain to back rebels fighting the al-Assad regime, saying the United States sought no role in the Syrian civil war.
When evidence of chemical weapons use emerged earlier this year, and Hezbollah fighters from Lebanon helped al-Assad's forces gain the upper hand, Obama agreed in June to provide military aid to the rebels.
The August attack clearly crossed a "red line" he declared earlier against chemical weapons use, prompting his decision for what he hoped would be an international military response against Syria.
However, Britain's Parliament voted against joining a military response, denying Obama a normally reliable ally. He then decided to seek authorization from Congress to provide political cover and buy time to build a broader international coalition.
Now legislators from both parties are threatening to oppose a resolution authorizing a military response, and Obama has asked for time to let the diplomatic process play out.
Kerry heads to Geneva on Thursday for talks with his Russian counterpart, who first offered his government's proposal Monday after Kerry earlier said Syria's turning over its chemical weapons was the only way to avoid a U.S. attack.
Syria agreed Tuesday to the Russian proposal, with Foreign Minister Walid Moallem saying his government was ready to disclose the location of its chemical weapons, halt production, and show its facilities to representatives of Russia, the United Nations, and other unspecified states.
At the same time, Russian President Vladimir Putin said Tuesday that "all of this will only mean anything if the United States and other nations supporting it tell us that they're giving up their plan to use force against Syria."
"You can't really ask Syria, or any other country, to disarm unilaterally while military action against it is being contemplated," Putin said in an interview with a Russian television network.
Obama: Threat of strikes still critical