American military officials say they do not believe the strike on Tuesday, which they said was carried out with a nerve agent, was necessarily unique. On March 30, panicky Syrian forces may have used a similar nerve agent in Hama Province, though American officials said they lacked forensic evidence to prove it. On March 25, the Syrians also mounted an attack using chlorine; its use in war is illegal under an international convention banning chemical weapons.

Having concluded that chemical weapons were used by Syrian forces, the next challenge for the Trump administration was to settle on a response. The military options were developed on Wednesday, and when they were narrowed down, the Al Shayrat airfield was in the cross hairs.

Equipped with bunkers for storing chemical munitions, the airfield had been built as a potential launching pad for attacks with chemical weapons — weapons that Mr. Assad was supposed to have given up as part of an agreement that was worked out by the United States and Russia.

Surveying the airfield, American war planners developed a list of 59 targets: aircraft, hardened plane shelters, radars, an air defense system, ammunition bunkers and petroleum storage sites. One Tomahawk cruise missile was fired at each of the 59 targets, and the Pentagon asserted that each hit its mark. An additional missile aborted after launch and fell into the Mediterranean.

One American official who spoke separately from the briefing estimated that 20 to 25 Syrian warplanes were destroyed in the attack, at 3:40 a.m. local time, four hours after President Trump’s order to go ahead was relayed to the Central Command. The runway was not a target.

The strike was aimed at avoiding the 12 to 100 Russian pilots, maintenance and other military personnel who manned a helicopter unit at different parts of the base, and to avoid striking Russian aircraft. American officials said they had no independent information on possible casualties but were confident that Russians were not among them.

The presence of the Russians is just one factor that is leading American intelligence officials to investigate if Moscow was complicit, disinterested or ignorant of the Syrian government’s use of a covert chemical arsenal.