There are two ways the forces could function as a military deterrent: either (1) the U.S. deployment would be large enough to serve as an effective military barrier to a future Syrian military offensive or (2) the U.S. deployment would serve as a "tripwire" to ensure that such a Syrian offensive would trigger a large American military intervention to oppose it.
CENTERFORSECURITYPOLICY: Center for Security Policy | What hazards along the road to Damascus? Mission impossible