The Americans wanted to send a specific signal by striking only the airfield that a Syrian Su-22 warplane had used for its mission on Tuesday to drop a chemical bomb in the middle of the town of Khan Sheikhoun, in southern Idlib Province.
Before the American attack could go forward, however, American intelligence officials had to satisfy senior commanders — and, presumably, President Trump — that it had the culprit. The evidence was abundant, and American intelligence analysts concluded they had “high confidence” in their assessments.
The Americans had tracked the Syrian jet as it took off from the Al Shayrat airfield and dropped a bomb in the middle of a street. The time of the chemical attack, just before 7 a.m., correlated with reports that residents were exhibiting signs of having been subjected to nerve agent. The crater from the bomb showed staining that experts associated with a shell filled with chemical agents.
American intelligence officials also suspect that an attempt might have been made to frustrate efforts to gather evidence of a chemical assault. After victims were rushed to a hospital, a small drone appeared overhead before disappearing. About five hours later, the drone returned and another airstrike hit the medical center; American officials do not know if the drone or the second strike was launched by Syria or Russia.
The shifting fortunes on the battlefield may explain why the Assad government mounted its largest chemical weapons attack since August 2013. In recent weeks, rebel forces have pushed to connect the areas they controlled in Hama and Idlib Provinces. The Syrian government’s control of the Hama airfield was at risk; it was being used by the Assad government as a helicopter base and, it is suspected, as a factory for some of the barrel bombs Syria’s forces had used to deadly effect.
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.
As long as Syrian forces do not use chemical weapons again, American officials have signaled that Mr. Trump’s first use of force against Mr. Assad’s military is likely to have been a shot heard around the world — but also a riposte that will not be repeated.
Limiting the effect of even a narrow operation, however, may be difficult. One potential danger is that Shiite militias backed by Iran, including Shiite fighters in Iraq, might try to retaliate against American troops.
Another is that Russian and American relations may deteriorate to the point that the procedures the two nations use to notify each other about air operations in Syria will be suspended, raising the risk of an inadvertent confrontation.
A more fundamental question is whether the Trump administration will now pursue a diplomatic strategy to quell the fighting in Syria, which has attracted thousands of militants, including many of the volunteers who have joined the Islamic State.
Cliff Kupchan and Ayham Kamel of the Eurasia Group, a political risk consultancy, predicted that Mr. Assad would avoid a direct confrontation with the United States in the near term, calculating that he has enough aircraft, barrel bombs, missiles and troops to continue his fight against the rebels without resorting to poison gas.
Mr. Assad and his aides, they wrote in an assessment, “will probably steer away from any escalation that would lead the international community to recommit itself to a regime change policy.”
Mr. Trump’s intervention against the Assad government may be over for now, but the Syrian war appears certain to go on.