Damascus, the capital, should be the second target. But unlike Aleppo, it can’t be easily reached from a Turkish base. It could, however, be supplied from Dara’a, which is 70 miles from Damascus and less than five from the Jordanian border. It has been at the forefront of opposition to Mr. Assad. Working with Jordan, the United States could create a second corridor to Dara’a, which could serve as the southern base for the insurgency. On Wednesday, by bombing a military complex, the rebels demonstrated their ability to strike in the heart of Damascus — though they have not yet been able to do so on a sustained basis.

To prevent Mr. Assad from staging a devastating response, the American-backed alliance would have to create a countrywide no-fly zone, which would first require taking apart Syrian air defenses. Mr. Assad has been using jets and helicopters to fight the rebels; a no-fly zone would quickly ground his entire air force. The zone could then be extended to provide the kind of close air support that NATO warplanes provided to rebel fighters in Kosovo and Libya.

While our allies could take the lead in maintaining the no-fly zone, it is necessary in Syria, as in Libya, for America to take the lead in establishing it; only our Air Force and Navy have the weaponry needed to dismantle Syria’s Russian-designed air defenses with little risk.