WASHINGTON • The cruise missile strike that destroyed at least 20 warplanes in Syria on Friday was devised by US war planners as a one-time operation to deter President Bashar al-Assad from using his secret stockpile of chemicals again.
Military officials said it was never intended to be the leading edge of a broader campaign to dislodge Mr Assad from power, or force a political settlement in a country that has been ripped apart by six years of bloody civil war.
But the question for the Pentagon is whether this 21st-century equivalent of a shot across the bow will ensure that poisonous gas will no longer be among the many scourges that plague Syria, or whether it will gradually draw the United States into a multi-sided military tug of war over Syria's future.
If there is one description that summed up the plan, which was developed at the headquarters of the US Central Command in Tampa, Florida, it is "proportional". Details of the plan were described to reporters at a briefing on Friday by senior military officials who spoke on the condition of anonymity.
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 on the town of Khan Sheikhoun, in southern Idlib province.
US military officials said they do not believe Tuesday's strike, which they said was carried out with a nerve agent, was necessarily unique... A more fundamental question is whether the Trump administration will pursue a diplomatic strategy to quell the fighting in Syria.
Before the US attack could go forward, however, US intelligence officials had to satisfy senior commanders - and, presumably, President Donald Trump - that it had the culprit. The evidence was abundant, and US intelligence analysts concluded they had "high confidence" in their assessments.
The Americans had tracked the Syrian jet as it took off from the Shayrat airfield and dropped a bomb in the middle of a street. The time of the chemical attack, just before 7am, 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.
US 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 airs trike hit the medical centre; US 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.
US military officials said they do not believe Tuesday's strike, 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 US officials said they lacked forensic evidence to prove it. On March 25, Syria also mounted an attack using chlorine; its use in war is illegal under a 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 Shayrat airfield was in the crosshairs.
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 worked out by the US and Russia.
US war planners developed a list of 59 targets: aircraft, hardened plane shelters, radars, an air defence system, ammunition bunkers and fuel 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 US official who spoke separately from the briefing estimated 20 to 25 Syrian warplanes were destroyed in the attack. The runway itself 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. US officials said they had no independent information on possible casualties but were confident that Russians were not among them.
A more fundamental question is whether the Trump administration will pursue a diplomatic strategy to quell the fighting in Syria.
Analysts Cliff Kupchan and Ayham Kamel of the Eurasia Group, a political risk consultancy, predicted that Mr Assad would avoid a direct confrontation with the US 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".