Mr. Mattis was working to slow the move toward a military response, concerned that a missile strike could spark a wider conflict between Russia, Iran and the West.
And Mr. Trump sent mixed signals about timing. On Wednesday, he warned Russia on Twitter that missiles “will be coming, nice and new and ‘smart!’ ” But the next day, he added: “Never said when an attack on Syria would take place. Could be very soon or not so soon at all!”
At a news conference on Saturday morning, Prime Minister Theresa May of Britain said that the strikes had been “the right thing to do,” in part for the “operational security” of those carrying them out.
Assad absorbed another blow.
The airstrikes sent an unambiguous message to Mr. Assad, and it was not clear that it would change his thinking. He remained firmly in power thanks to the support of Russia and Iran.
Mr. Assad has essentially been under siege since the Syrian civil war began more than seven years ago. In that time, he has dealt with the war, airstrikes, sanctions, Islamic State militants, a variety of rebel groups and a crumbling economy.
As Syrian state news media reported that many of the missiles had been intercepted, the Syrian presidency’s Twitter account posted a video that appeared to show Mr. Assad showing up for just another day at the office.
The events depicted could not be independently verified.
Capitol Hill remained divided.
The reaction in Washington was divided along party lines, with the strikes bringing praise from Republicans and criticism from Democrats.
“The Butcher of Damascus learned two lessons tonight the hard way,” Senator Tom Cotton, Republican of Arkansas, said in a statement. “Weapons of mass destruction won’t create a military advantage once the United States is done with you and Russia cannot protect its clients from the United States.”
Senator Tim Kaine, Democrat of Virginia, complained that Mr. Trump had not sought permission from Congress. Proceeding with the strikes, he said, was “illegal and — absent a broader strategy — it’s reckless.”
Russia responded with angry rhetoric, so far.
Russia has called for an urgent meeting of the United Nations Security Council, and offered some harsh warnings before the attack. But the speed and the tone of the Russian reaction on Saturday, stressing that the attack had not resulted in a direct confrontation and was rather limited, suggested almost relief on the part of the Kremlin.
According to Russian state news media, President Vladimir V. Putin condemned the missile strikes as an “act of aggression against a sovereign state” and against the United Nations Charter.
Gen. Valery V. Gerasimov, the chief of staff for the armed forces, had warned that Russia would “take retaliatory measures,” but he included an important caveat: Russia would attack missiles and the platforms from which they were launched only in the event that Russian military personnel were placed in danger.
In another sign of the lack of urgency, the Federation Council, the upper house of the Russian Parliament, stated that it would discuss the airstrikes “next week.”
Britain’s leader avoided a conflict at home.
Mrs. May has said she believed there was a need to send a strong message about the use of chemical weapons, but she also had compelling diplomatic and political reasons to support the United States — and to carry out the strikes as soon as possible.
One imperative was the desire to reciprocate the support London has received from the United States in the dispute with Russia over the poisoning of a former spy, Sergei V. Skripal, and his daughter, Yulia S. Skripal, on British soil.
Mrs. May, who made an explicit connection between the airstrikes in Syria and the poisoning of the Skripals, benefited from the timing of the airstrikes, two days before lawmakers were to return from vacation. While not obligated to consult Parliament, she may have felt constrained to do so and could easily have lost a vote on a strike, as her predecessor David Cameron did in 2013.
Britain also wants to prove its use as an ally to Mr. Trump at a time when its international influence is under question because of its withdrawal from the European Union, and when it hopes to strengthen trade ties with the United States.
France saw a chance to act after a red line was crossed.
President Emmanuel Macron had prepared his nation for this moment: He had discussed the possibility of airstrikes and made clear early in his presidency that the use of chemical weapons was a red line.
While pressing for a military response to the use of chemical weapons, Mr. Macron has also said he wants to work on a peace deal for the region, creating a dual-pronged strategy that has support in France. Mr. Macron has cultivated a closer relationship with Mr. Trump than other Western leaders, but he has also reached out to Mr. Putin.
He will visit both presidents in the coming weeks: Mr. Trump at the end of April and Mr. Putin in May.