If Assad succeeds in crushing the opposition or otherwise maintains control over most of Syria, Iran will have a massive new degree of influence over Iraq, Syria and Lebanon in a polarized Middle East divided between Sunni and Shiite. Minorities will be steadily driven into exile. This would present serious risks for Israel, weaken Jordan and Turkey and, most important, give Iran far more influence in the Persian Gulf, an area home to 48 percent of the world’s proven oil reserves.

If Washington arms the rebels and they still lose, the United States will at least have shown its willingness to make decisions and honor its commitments. It will have shown it will make good on its words and support its allies.

More advance transfers of U.S. arms, such as the antitank guided missiles and surface-to-air missiles that the rebel commanders say can shift the balance, could also be supplied and funded by our gulf allies. They do not have to be cutting-edge U.S. systems, and the rebels already have some Chinese and Russian man-portable surface-to-air missiles, as well as systems that could be a major threat to civilian targets, should they fall into extremist hands. It is unlikely the United States can control such transfers from friendly Arab gulf states if we do not supply the rebels, and it is far more likely that we can have a major influence on which faction gets such arms if we work with the rebels — particularly now that Qatar seems more willing to cooperate with the United States and Saudi Arabia.