President Obama has at last decided to deliver military support to Syria’s rebels, though the quantity and quality of any U.S. arms deliveries remain to be seen. It’s a move that, if made 18 months ago, might have decisively tilted the civil war against the regime of Bashar Assad and prevented the emergence of the Al-Qaida-linked extremist forces now active around the country. As it is, the action may be too small and come too late to achieve Obama’s stated goal of removing Assad from power.

The president’s hand was forced in part by strong evidence that the Assad regime had crossed a U.S. “red line” by using deadly sarin gas against rebels and civilians on multiple occasions; the White House’s announcement that chemical weapons had been used came months after Britain and France reached that conclusion. But Obama was also responding to desperate pleas from Gen. Salim Idriss, the commander of the rebel force that the United States says it supports. In a phone call to a senior State Department official and in a message to Washington, the general warned of preparations by the regime to mount a new offensive in Syria’s largest city, Aleppo, aimed at driving the rebels from districts they control.

Whether the United States and its allies can help prevent what could be a disastrous reverse to the anti-Assad cause will depend on how quickly arms supplies are delivered and whether they include the materiel Idriss says he most needs: not just ammunition for rifles and machine guns but also antitank rockets and anti-aircraft systems. But merely preventing opposition-held districts in Aleppo from being overrun will not achieve U.S. aims in Syria.

Administration officials appear to hope that small-scale U.S. aid, combined with the evidence of chemical-weapons use, will finally persuade ­Russia to push the regime into a negotiated solution requiring Assad’s departure. It won’t: Moscow quickly rejected the U.S. chemical weapons dossier and will surely continue its own weapons deliveries to the regime.

Only if the balance of the war shifts decisively to the side of the rebels will an acceptable political settlement be possible. That will almost certainly require a more robust U.S. intervention.