But a rebel victory would also be extremely dangerous for the United States and for many of its allies in Europe and the Middle East. That’s because extremist groups, some identified with Al Qaeda, have become the most effective fighting force in Syria. If those rebel groups manage to win, they would almost certainly try to form a government hostile to the United States. Moreover, Israel could not expect tranquillity on its northern border if the jihadis were to triumph in Syria.

Things looked far less gloomy when the rebellion began two years ago. At the time, it seemed that Syrian society as a whole had emerged from the grip of fear to demand an end to Mr. Assad’s dictatorship. Back then, it was realistic to hope that moderates of one sort or another would replace the Assad regime, because they make up a large share of the population. It was also reasonable to expect that the fighting would not last long, because neighboring Turkey, a much larger country with a powerful army and a long border with Syria, would exert its power to end the war.

As soon as the violence began in Syria in mid-2011, Turkey’s prime minister, Recep Tayyip Erdogan, loudly demanded that it end. But instead of being intimidated into surrender, Mr. Assad’s spokesmen publicly ridiculed Mr. Erdogan, while his armed forces proceeded to shoot down a Turkish fighter jet, before repeatedly firing artillery rounds into Turkish territory and setting off lethal car bombs at a Turkish border crossing. To everyone’s surprise, there was no significant retaliation. The reason is that Turkey has large and restless minority populations that don’t trust their own government, which itself does not trust its own army. The result has been paralysis instead of power, leaving Mr. Erdogan an impotent spectator of the civil war on his doorstep.

Consequently, instead of a Turkey-based and Turkish-supervised rebellion that the United States could have supported with weapons, intelligence and advice, Syria is plagued by anarchic violence.

The war is now being waged by petty warlords and dangerous extremists of every sort: Taliban-style Salafist fanatics who beat and kill even devout Sunnis because they fail to ape their alien ways; Sunni extremists who have been murdering innocent Alawites and Christians merely because of their religion; and jihadis from Iraq and all over the world who have advertised their intention to turn Syria into a base for global jihad aimed at Europe and the United States.