[Read statements by France and Britain, and a declassified French document on the airstrikes.]

For Mr. Macron, the move also had the salutary effect of shoring up his position as a broker between Russia and the United States and the European Union. Just hours before launching the strike, Mr. Macron was on the telephone with President Vladimir V. Putin of Russia to discuss Syria. And both Mr. Macron and his aides have said that he still plans to go ahead with a state visit to Moscow at the end of May.

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.