KILIS, Turkey — Turkey is relying on a newly reconfigured, 20,000-member American-trained force with three army corps as it tries to carve out a buffer zone within Syria. The force has already taken 16 casualties in two weeks of fighting on the front lines.

But the soldiers are not Turks. Rather they are the mostly Arab fighters of the Free Syrian Army, once trained and assembled by the C.I.A. and Western allies to oust President Bashar al-Assad of Syria.

Since the routing of the Islamic State, alliances in Syria have been scrambled. The latest round of the conflict, in fact, features two American-trained forces fighting each other.

The Free Syrian Army, out of favor with the United States and badly depleted after seven years of fighting on multiple fronts, has long had common cause with Turkey, whose incursion has angered the Americans.