At the same time, Mr. Macron is trying to reinforce France’s position as an enforcer of international treaties, which includes the Chemical Weapons Convention that 192 countries have signed.
Mrs. May is in a more precarious position, with a tense standoff developing with Russia over the poisoning of a former Russian spy, Sergei V. Skripal, and his daughter, Yulia Skripal, with a weapons-grade nerve agent. Mrs. May has been under pressure to respond forcefully to Moscow for the attack, which exposed hundreds of citizens in southwest England to the deadly substance.
On Saturday, the British leader described the airstrikes in Syria as “right and legal,” drawing an explicit distinction between those and the poisoning of the Skripals — the first use of chemical weapons in Europe since World War II.
Mrs. May also 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. Another imperative for Britain was to reciprocate the support that London has received from the United States in the dispute with Russia over the poisoning.
“I don’t think she had much choice,” said Justin Bronk, a research fellow for air power at the Royal United Services Institute, a defense and security focused research institute in London. “By announcing the strikes through Twitter, President Trump made this a personal call to action with his own image and credibility at stake, and he’s an openly transactional president.”
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 as it hopes to strengthen trade ties with the United States.
On Saturday, as reaction to the strikes rolled in from around the world, Mr. Trump wrote on Twitter: “A perfectly executed strike last night. Thank you to France and the United Kingdom for their wisdom and the power of their fine Military. Could not have had a better result. Mission Accomplished!”
Opinion polls suggested that the British public’s support for strikes was lukewarm. So in backing Mr. Trump’s airstrikes without seeking prior approval from lawmakers, Mrs. May took a political risk, albeit one that should be manageable unless the conflict escalates.
The prime minister did come in for criticism. The leader of the opposition Labour Party, Jeremy Corbyn, criticized Mrs. May, arguing that “bombs won’t save lives or bring about peace.”
Scotland’s first minister, Nicola Sturgeon, faulted Mrs. May for failing to follow recent precedent and seek parliamentary approval. But Mrs. May brushed aside the protests, presenting her decision to order “limited, targeted and effective” strikes against Syria as vital in deterring future use of chemical weapons.
London has looked on with concern as Mr. Macron has cultivated close ties with Mr. Trump, while Mrs. May’s relationship with the White House has been more complicated and tense.
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Mr. Macron, for his part, faced criticism on the far left and the far right for his decision to join the attack on Syrian targets. The leader of the far left France Insoumise party, Jean-Luc Mélenchon, accused Mr. Macron on Twitter of attacking Syria without proof of chemical weapons use and without a United Nations mandate, a European Union agreement or a vote of the French Parliament.
“This is a North American adventure of revenge, an irresponsible escalation,” Mr. Mélenchon said.
On the extreme right, Marine Le Pen, the leader of the National Front, said France had lost a chance to “appear on the international scene as an independent power.” The party’s deputy leader, Nicolas Bay, called Mr. Macron “a vassal” of the United States.
For both Mrs. May and Mr. Macron, the strikes made good on their respective predecessors’ pledges to retaliate over Syria’s use of chemical weapons. In 2013, when the government of President Bashar al-Assad was suspected of using chemical weapons on his own people, both François Hollande, then president of France, and David Cameron, then Britain’s prime minister, said he had cross their red lines.
But when it came down to launching the missiles, Mr. Cameron failed to secure Parliament’s approval, and Mr. Hollande backed down when it became clear that President Barack Obama had second thoughts because he was afraid of being drawn into a larger fight with Mr. Assad.
It was easier for the leaders to sign on to this missile attack, as there was no discussion about a far-reaching military campaign or about regime change. And there was an explicit effort to avoid hitting bases where troops from Russia and Iran, allies of Mr. Assad, might be amassed.
The United Nations Security Council held an emergency meeting on Saturday but rejected a Russian resolution condemning the missile strikes in Syria. European Union foreign ministers, meanwhile, planned to meet on Monday to tackle the tense situation in Syria.
A draft statement, written before the strikes, proposed looking at fresh sanctions on Syria, including blacklisting more people over the development and use of chemical arms.
The NATO secretary general, Jens Stoltenberg, later expressed support for the strikes by its three member states, saying that they “will reduce the regime’s ability to further attack the people of Syria with chemical weapons.”
Mr. Stoltenberg said in a statement that Syria’s use of chemical weapons was “a clear breach of international norms and agreements.”
He added, “NATO considers the use of chemical weapons as a threat to international peace and security, and believes that it is essential to protect the Chemical Weapons Convention,” which Syria signed in 2013, leading to a ban in the country on the manufacture or use of such weapons.