ALEPPO, Syria

A YOUNG fighter for the Free Syrian Army sat at a checkpoint on a couch taken from an abandoned house. He cradled his Kalashnikov and waited on the empty street for a car to inspect, or a pedestrian to pat down. If only the future of Syria would reveal itself to him as easily.

The rebels in the Free Syrian Army don’t doubt that they will drive President Bashar al-Assad from power — eventually — but they have no idea what will happen afterward: Democracy? An Islamic republic? An Islamic dictatorship? The fighters I met on a recent visit here were unable to articulate any long-term political vision.

While the young rebel sat at his checkpoint, and while Americans continue to debate whether to intervene in Syria or just look the other way, Islamist militants are exploiting the uncertainty here. They have a clear mission: imposing an Islamist state in place of Mr. Assad.

“The people who believe in a strict Islam will do anything, fight anybody, do anything for Islam,” a barber who recently reopened his shop told me. “They are like the U.S. Special Forces. They like death more than life.”