Beyond the immediate question of whether the new strikes actually accomplished the stated goal of diminishing Syria’s capacity to make and use chemical agents, the attack posed the risk of drawing the United States more deeply into a conflict in which Russia and Iran have more invested than ever in keeping Mr. Assad in power.
The United States is “locked and loaded” to strike again if Mr. Assad is believed to renew his use of chemical weapons, Nikki R. Haley, the American ambassador to the United Nations, told the Security Council on Saturday at an emergency meeting called by Russia.
“We are confident that we have crippled Syria’s chemical weapons program. We are prepared to sustain this pressure, if the Syrian regime is foolish enough to test our will,” Ms. Haley said.
She said that Russia had failed to abide by a 2013 promise to ensure that Syria got rid of its chemical weapons stockpiles.
“While Russia was busy protecting the regime, Assad took notice,” she said. “The regime knew that it could act with impunity, and it did.”
The Pentagon provided no immediate evidence that the sites that were struck were producing substances covered by the 2013 agreement between Russia and the United States to eliminate Syria’s chemical arms.
Defense Secretary Jim Mattis said on Friday that the American government was confident that Syrian forces had used chlorine in the deadly attack on civilians last Saturday in Douma, but did not provide evidence.
The White House has cited photographs and videos from Douma to make the case, and has dismissed alternative explanations from the Syrian and Russian governments. It said that the nerve agent sarin may have been used in addition to chlorine.
Chlorine, as a commercially available substance, was not included in the 2013 agreement. But the Chemical Weapons Convention, which Syria joined as part of the 2013 deal, prohibits the use of any chemical as a weapon.
The American-led strikes on Syria jolted residents of Damascus, the capital, from their beds as their walls and windows shook. American officials said the offensive was to punish Syria for the Douma attack, which left scores dead.
A group from the Organization for the Prohibition of Chemical Weapons, which had announced a fact-finding mission to determine if chemical weapons were used in the Douma attack, arrived in Damascus on Saturday morning, the group said in a statement.
A statement by the Syrian military said 110 missiles had been fired in the American-led strike. Three people were injured in Homs, it said. Videos from Damascus showed Syrian air defense missiles launching into a dark night sky, and the Russian military said that at one Syrian air base, all 12 cruise missiles that targeted the site had been shot down.
Defense Department officials batted down those claims, saying that the entire American-led operation was over and the targets were destroyed before Syria launched any of the 40 missiles it fired into the air.
“Taken together, these attacks were able to overwhelm the Syrian air defense system,” Lt. Gen. Kenneth F. McKenzie, the Pentagon’s Joint Staff director, said at a news conference. “None of our aircraft involved were successfully engaged by Syrian defense forces.”
He added that the barrage of missiles had hit their targets within a couple of minutes at most. He said that all three targets had been destroyed, and that all warplanes had returned safely to base.
But the strikes were limited, with an eye toward making sure they did not draw retaliation from Russia and Iran and set off a wider conflict. For that reason, Mr. Assad may still be able to use chemical agents in the future.
“I would say there’s still a residual element of the Syrian program that’s out there,” General McKenzie said. “I’m not going to say that they’re going to be unable to continue to conduct a chemical attack in the future. I suspect, however, they’ll think long and hard about it.”
In a statement, the British government said that Prime Minister Theresa May, President Emmanuel Macron of France and Mr. Trump had agreed in separate phone calls that the military strikes had been a success, “sending a clear message that the use of chemical weapons can never become normalized.”
But the limited nature of the strikes left some members of Congress and other observers underwhelmed.
“I fear that when the dust settles, this strike will be seen as a weak military response and Assad will have paid a small price for using chemical weapons yet again,” Senator Lindsey Graham, Republican of South Carolina and a member of the Armed Services Committee, said in a statement. “Assad has likely calculated a limited American strike is just the cost of doing business.”
Randa Slim, an analyst at the Middle East Institute, tweeted: “If this is it, Assad should be relieved.”
Sure enough, early Saturday morning, Mr. Assad’s office posted a video that appeared to show him strolling into work in a suit and tie and carrying a briefcase as if nothing had happened.
Condemnation of the strikes from Syria’s allies was swift.
“The United States and its allies continue to demonstrate blatant disregard for international law,” said the Russian ambassador to the United Nations, Vasily A. Nebenzya, on Saturday. “It’s time for Washington to learn that the international code of behavior regarding the use of force is regulated by the United Nations Charter,” he added.
Ayatollah Ali Khamenei, the supreme leader of Iran, called the strikes “a crime” and the leaders of the United States, France and Britain “criminals.”
“But they will not benefit from this attack, just as they committed similar crimes over the past years during their presence in Iraq, Syria and Afghanistan and did not benefit from them,” he said.
There were no signs of immediate retaliation, suggesting that Mr. Assad and his allies planned to weather the storm, perhaps in the belief that the United States was mostly concerned with avoiding deeper involvement.
“If I were Assad, I would be thinking, ‘Let them get it out of their system. Things are still trending in the right direction today,’ ” said Faysal Itani, a senior fellow at the Atlantic Council. “Nothing that Trump said on television really touched on the Syrian conflict.”
In announcing the strikes Friday night, Mr. Trump suggested that more American action could be on the way. “We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents,” Mr. Trump said. Other officials, however, said the United States and its allies were done for now.
Saturday’s strikes were more extensive than those Mr. Trump launched in the wake of another reported chemical attack last year, involving nearly double the number of missiles. But much has changed in Syria in the meantime to make Mr. Assad and his allies more secure.
The rebels who once threatened his control have been routed from all of Syria’s major cities, and even from smaller strongholds like Douma, the last town they held near Damascus, which they surrendered after the reported chemical attack last weekend.
Meanwhile, the war has further shattered Syria, and international powers including the United States, Russia, Turkey, Israel and Iran — along with militant groups including Hezbollah — have intervened to fight for their interests.
Iran and Russia have expanded their military reach. Russia has a presence on most Syrian military bases, and its air force has been essential to Mr. Assad’s recent advances. Iran has used the chaos of war to strengthen its proxies to deter and possibly confront Israel.
The United States still has about 2,000 troops in eastern Syria working with a Kurdish-led militia to fight the jihadists of the Islamic State. But with the militants now nearly defeated, American officials have started thinking about when to withdraw. Before the suspected chemical weapons attack in Douma, Mr. Trump had said he wanted to bring them home soon.
While the United States called for Mr. Assad to leave power early in the conflict and gave cash and arms to the rebels who sought to overthrow him, it has more recently resigned itself to his remaining in power. That was partly because it feared the vacuum that could emerge if Mr. Assad’s government collapsed, and partly because it was clear that Russia and Iran were willing to invest more in winning than the United States was.
So Saturday’s strikes remained focused on punishing him for using chemical weapons. Last year’s strikes had the same goal, but only succeeded for a limited time.
“Military interventions have a shelf life,” said Andrew J. Tabler, a fellow at the Washington Institute for Near East Policy who studies Syria. “For a certain time, it prevented them from using chemical weapons, but after a while it dissipated. So we’ll have to wait to see how this attack is different.”
Mr. Trump made it clear when announcing the strikes that he does not consider it the job of the United States to fix problems in the Middle East.
“No amount of American blood or treasure can produce lasting peace and security in the Middle East,” he said. “It’s a troubled place. We will try to make it better, but it is a troubled place.”
He did, however, speak of working closely with American allies in the region such as Saudi Arabia, the United Arab Emirates, Egypt and Qatar to ensure that Iran did not capitalize on the defeat of the Islamic State.
Still, such an international coalition would be starting years behind what Russia and Iran have already done, and some of the Arab allies Mr. Trump mentioned are not even speaking to each other. Mr. Trump has so far shown little penchant for the kind of strategic alliance-building it would take to change the direction of the war.
Some Syrians, however, took solace in the sight of smoke rising from Mr. Assad’s military bases.
“For many Syrians, the fact that their butcher was punished is incredibly cathartic,” Mouaz Moustafa, the executive director of the Syrian Emergency Task Force, which opposes the government, wrote on Twitter. “But war continues.”