The presence of rebel fighters in Syria that were trained and supported by Al Qaeda poses a serious problem for the United States and Western allies. The Nusra Front, an offshoot of Al Qaeda in Iraq, has become one of the most effective forces fighting against President Bashar al-Assad.

The fear is that the group could hijack the revolution and emerge as the dominant force in Syria after Mr. Assad is ousted from power. Obama administration officials have been increasingly frank about this threat, along with the possibility that sectarian conflicts among the country’s Sunni, Alawite, Christian and other groups may well rage on after Assad.

There are no easy answers, and no one believes that Washington, or any external power, can dictate the outcome. But President Obama still needs to provide a clearer picture of how he plans to use American influence in dealing with the jihadi threat and the endgame in Syria.

Mr. Obama has blacklisted the Nusra Front as a terrorist organization, which would make it illegal for Americans to have financial dealings with it. It makes sense to isolate the group and try to dry up its resources, but the designation by itself isn’t sufficient. American officials have to make a case directly to the countries or actors that are believed to be most responsible, either directly or as a conduit, for the weapons and other assistance to the Nusra Front: Qatar, Saudi Arabia, Turkey and Iraqi Kurdistan. However much they may want to see Mr. Assad fall, they play a deadly game in empowering any affiliate of Al Qaeda, which though weakened, is dedicated to global jihad and the violent overthrow of Sunni monarchies.