The Marines will join the approximately 500 U.S. troops already in Syria, such as these U.S. forces on the western outskirts of the northern Syrian city of Manbij on Sunday. Delil Souleiman /AFP/Getty Images hide caption The Marines will join the approximately 500 U.S. troops already in Syria, such as these U.S. forces on the western outskirts of the northern Syrian city of Manbij on Sunday.