With Navy ships on standby in the Mediterranean Sea ready to launch their cruise missiles, Obama said he had decided the United States should take military action and that he believes that as commander in chief, he has "the authority to carry out this military action without specific congressional authorization."
At the same time, he said, "I know that the country will be stronger if we take this course and our actions will be even more effective." His remarks were televised live in the United States as well as on Syrian state television with translation.
Congress is scheduled to return from a summer vacation on Sept. 9, and in anticipation of the coming debate, Obama challenged lawmakers to consider "what message will we send if a dictator can gas hundreds of children to death in plain sight and pay no price."
The president didn't say so, but his strategy carries enormous risks to his and the nation's credibility, which the administration has argued forcefully is on the line in Syria. Obama long ago said the use of chemical weapons was a "red line" that Assad would not be allowed to cross with impunity.
Nor would the White House say what options would still be open to the president if he fails to win the backing of the House and Senate for the military measures he has threatened.
Only this week, British Prime Minister David Cameron suffered a humiliating defeat when the House of Commons refused to support his call for military action against Syria.
Halfway around the world, Syrians awoke Saturday to state television broadcasts of tanks, planes and other weapons of war, and troops training, all to a soundtrack of martial music. Assad's government blames rebels in the Aug. 21 attack, and has threatened retaliation if it is attacked.
Russian President Vladimir Putin, saying he was appealing to a Nobel Peace laureate rather than to a president, urged Obama to reconsider. A group that monitors casualties in the long Syrian civil war challenged the United States to substantiate its claim that 1,429 died in a chemical weapons attack, including more than 400 children.
By accident or design, the new timetable gives time for U.N. inspectors to receive lab results from the samples they took during four days in Damascus, and to compile a final report. After leaving Syria overnight, the inspection team arrived in Rotterdam a few hours before Obama spoke.
The group's leader was expected to brief Secretary-General Ban Ki-moon on Sunday.
Administration officials said Obama appeared set on ordering a strike until Friday evening. After a long walk in near 90-degree temperatures around the White House grounds with Chief of Staff Denis McDonough, the president told his aide he had changed his mind.
These officials said Obama initially drew pushback in a two-hour session attended by Vice President Joe Biden, Defense Secretary Chuck Hagel, Director of National Intelligence James Klapper, CIA Director John Brennan, national security adviser Susan Rice and homeland security adviser Lisa Monaco. They declined to say which of the participants had argued against Obama's proposal.
Whatever Congress ultimately decides, the developments marked a stunning turn.
France is Obama's only major foreign ally to date for a strike, public polling shows support is lukewarm in the United States, and dozens of lawmakers in both parties have signed a letter urging Obama not to act without their backing. Outside the gates of the White House, the chants of protesters could be heard as the president stepped to a podium set up in the Rose Garden.
Had he gone ahead with a military strike, Obama would have become the first U.S. leader in three decades to attack a foreign nation without mustering broad international support or acting in direct defense of Americans. Not since 1983, when President Ronald Reagan ordered an invasion of the Caribbean island of Grenada, has the U.S. been so alone in pursuing major lethal military action beyond a few attacks responding to strikes or threats against its citizens.
By day's end Saturday, the White House had sent Congress a draft of a resolution, crafted by the White House, to authorize Obama to use military force.
The draft does not lay out a specific timeline or course of military action, instead giving Obama approval to use the military as he determines "necessary and appropriate" to meet the objective of preventing future chemical weapons use. But in an overture to the limited scope of the strike Obama has said he's considering, the draft affirms the administration's view that ultimately, only a political solution can resolve the crisis in Syria.
House Speaker John Boehner, R-Ohio, said he expected the House to consider the measure the week of Sept. 9. Senate Majority Leader Harry Reid, D-Nev., said he, too, will hold a vote no later than the week of Sept. 9, with public hearings starting next week.
Republicans generally expressed satisfaction at Obama's decision to seek congressional support, and challenged him to make his case to the public and lawmakers alike that American power should be used to punish Assad.
"We are glad the president is seeking authorization for any military action in Syria in response to serious, substantive questions being raised," Boehner and other House Republican leaders said in a joint statement.
New York Republican Rep. Peter King was among the dissenters, strongly so. "President Obama is abdicating his responsibility as commander in chief and undermining the authority of future presidents," he said. "The president doesn't need 535 members of Congress to enforce his own red line."
For now, it appeared that the administration's effort at persuasion was already well underway.
The administration plunged into a series of weekend briefings for lawmakers, both classified and unclassified, and Obama challenged lawmakers to consider "what message will we send to a dictator" if he is allowed to kill hundreds of children with chemical weapons without suffering any retaliation.
At the same time, a senior State Department official said Secretary of State John Kerry spoke with Syrian Opposition Coalition President Ahmed Assi al-Jarba to underscore Obama's commitment to holding the Assad government accountable for the Aug. 21 attack.
Obama said Friday he was considering "limited and narrow" steps to punish Assad, adding that U.S. national security interests were at stake. He pledged no U.S. combat troops on the ground in Syria, where a civil war has claimed more than 100,000 civilian lives.
In Syria, some rebels expressed unhappiness with the president, one rebel commander said he did not consider Obama's decision to be a retreat. "On the contrary, he will get the approval for congress and then the military action will have additional credibility," said Qassem Saadeddine.
"Just because the strike was delayed by few days doesn't mean it's not going to happen," he said.
With Obama struggling to gain international backing for a strike, Putin urged him to reconsider his plans. "We have to remember what has happened in the last decades, how many times the United States has been the initiator of armed conflict in different regions of the world, said Putin, a strong Assad ally. "Did this resolve even one problem?"
Even the administration's casualty estimate was grist for controversy.
The Syrian Observatory for Human Rights, an organization that monitors casualties in the country, said it has confirmed 502 deaths, nearly 1,000 fewer than the American intelligence assessment claimed.
Rami Abdel-Rahman, the head of the organization, said he was not contacted by U.S. officials about his efforts to collect information about the death toll in the Aug. 21 attacks.
"America works only with one part of the opposition that is deep in propaganda," he said, and urged the Obama administration to release the information its estimate is based on.
Associated Press writers Julie Pace, Josh Lederman, Matthew Lee and Kimberly Dozier in Washington; Zeina Karam, Yasmine Saker and Karin Laub in Beirut; and Geir Mouslon in Berlin contributed to this report.
U.S. government assessment