WASHINGTON -- President Obama stunned the world and paused his march to battle on Saturday by asking Congress to give him authorization before he launches a limited military strike against the Syrian government in retaliation for a chemical weapons attack.
In an afternoon appearance in the Rose Garden, Mr. Obama said he had decided that the United States should use force but would wait for a vote from lawmakers, who are not due to return to town until Sept. 9. Mr. Obama said he believed he had the authority to act on his own, but he did not say whether he would if Congress rejects his plan.
"I'm prepared to give that order," Mr. Obama said. "But having made my decision as commander in chief based on what I am convinced is our national security interest, I'm also mindful that I'm president of the world's oldest constitutional democracy."
Going to war with the support of the people's representatives, he added, "I know the country will be stronger."
The president's announcement effectively dared Congress to either stand by him or, as he put it, allow President Bashar al-Assad of Syria to get away with murdering children. By asking lawmakers to weigh in, he is trying to break out of his box of isolation of the last week, in the face of deep skepticism at home and around the world about the strike. His decision indicates he does not want to go forward without Congress and the American public.
But it represents a major political gamble for a president with marginal command of Congress. Officials said he is likely to win support in the Senate, where leading Republicans quickly issued statements welcoming his decision, but the House is more of an open question given its strong current of antiwar sentiment in both parties.
The decision also means that the period of vacillation before a strike will extend until after Mr. Obama travels to St. Petersburg, Russia, for a summit meeting of the Group of 20 nations, a session that now seems certain to be dominated by the question of what to do about Syria.
President Vladimir V. Putin, the host of the meeting, has not only effectively blocked United Nations action, but on Saturday he suggested the American intelligence blaming Mr. Assad's government for the chemical attack was based on a ruse.
Opponents of military action hoped the extra time would give them a chance to stop another American intervention in a region that has entangled the country at great cost for a dozen years since the terrorist attacks of Sept. 11, 2001.
Supporters of a military strike worried that stretching out the decision process would make Mr. Obama look like he blinked and undercut American credibility.
"In the same presentation, he made a persuasive case for military action and then in a dramatic pivot put it at risk, too," said Aaron David Miller, a longtime Middle East adviser to multiple presidents and now at the Woodrow Wilson International Center for Scholars. It "shows just how concerned he is about being alone and his understanding of the realities that even a limited strike can be risky, and he wants to share the responsibility."
Mr. Obama spoke with the Democratic and Republican leaders of both houses before his announcement and said they agreed to schedule a debate and votes. The House leadership said it would stick to its schedule of resuming work the week of Sept. 9, but Senate leaders were considering coming back early, perhaps on Friday, for a weekend of debate, several senators said.
Either way, Congressional leaders hailed the move without committing to supporting his request for authorization.
"We are glad the president is seeking authorization for any military action in Syria in response to serious, substantive questions being raised," Speaker John A. Boehner said in a statement.
Senator Mitch McConnell, the Republican minority leader, likewise welcomed the decision. "The president's role as commander in chief is always strengthened when he enjoys the expressed support of the Congress," he said.
As a candidate, Mr. Obama said a president "does not have the power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation." But he did not ask Congress for permission when he backed a NATO military operation in Libya in 2011, unlike President George W. Bush who received legislative approval for wars in Afghanistan and Iraq.
The United States has carried out punitive raids without such Congressional authorization, including President Ronald Reagan's 1986 raid on Libya and President Bill Clinton's attacks in Afghanistan and Sudan in retaliation for the bombings of two American embassies in Africa in 1998.
But those were surprise attacks, and the current operation is one of the most heavily telegraphed punitive attacks in recent history.
Even as he made the request to Congress, Mr. Obama argued more forcefully than he ever had for military action, echoing some of the moral outrage expressed by Secretary of State John Kerry a day earlier. "What message will we send if a dictator can gas hundreds of children to death in plain sight and pay no price?" Mr. Obama said.
To shore up support, the White House assigned Mr. Kerry, Defense Secretary Chuck Hagel and other top officials to brief both parties in the Senate by telephone on Saturday and scheduled a classified briefing in person on Capitol Hill on Sunday for any lawmakers in town.
But the administration had not convinced everyone. Mr. Putin argued that it was "simply utter nonsense" to believe Syria's government would launch such an attack and challenged the United States to present any evidence to the United Nations.
"I am convinced that it is nothing more than a provocation by those who want to involve other countries in the Syrian conflict, who want to gain the support of powerful members in international affairs, primarily, of course the United States," Mr. Putin said in his first public remarks since reports of the chemical attack emerged. "I have no doubts about it."
United Nations inspectors left Syria on Saturday after four days of efforts to investigate the Aug. 21 attack. The inspectors were heading to The Hague with blood and urine samples taken from victims of the attack, as well as soil samples from areas where the attacks took place. The samples will be divided so each can be sent to at least two separate European laboratories for testing.
Angela Kane, the United Nations disarmament chief, briefed Secretary General Ban Ki-moon on Saturday and reported that inspectors were "able to conduct a wide range of fact finding activities," according to Martin Nesirky, the spokesman for the secretary general. Mr. Nesirky repeated that the inspectors' mandate was to find out whether chemical weapons were used, not who was responsible.
Mr. Nesirky said the inspection team would not transmit its report to Mr. Ban until it received laboratory results and he offered no estimate of how long that would take but added that "whatever can be done to speed up the process is being done." He added that Mr. Ban wants a political response. "A military solution is not an option," he said.
Obama administration officials argued that the United Nations findings would be redundant, since American intelligence had already concluded, based on human sources and electronic eavesdropping, that Mr. Assad's government was responsible for launching nerve agents in the eastern suburbs of Damascus.
Reporting was contributed by Anne Barnard from Beirut, Lebanon; Jackie Calmes, Michael R. Gordon and Jonathan Weisman from Washington; Steven Lee Myers from Moscow; and Marc Santora from New York.
This article originally appeared in The New York Times.