Indeed, it would be disastrous if President Bashar al-Assad’s regime were to emerge victorious after fully suppressing the rebellion and restoring its control over the entire country. Iranian money, weapons and operatives and Hezbollah troops have become key factors in the fighting, and Mr. Assad’s triumph would dramatically affirm the power and prestige of Shiite Iran and Hezbollah, its Lebanon-based proxy — posing a direct threat both to the Sunni Arab states and to Israel.

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. …

There is only one outcome that the United States can possibly favor: an indefinite draw.

By tying down Mr. Assad’s army and its Iranian and Hezbollah allies in a war against Al Qaeda-aligned extremist fighters, four of Washington’s enemies will be engaged in war among themselves and prevented from attacking Americans or America’s allies.