The U.S. has agreed to send about 40 special-operations troops to work alongside Turkish forces to fight Islamic State in northern Syria, U.S. officials said.
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The joint mission will take the forces east toward the northern Syrian town of Dabiq, a symbolically significant Islamic State stronghold. U.S. special-operations forces will operate as combat advisers and generally under the same guidelines as other special -operations forces are working inside Syria, U.S. officials said.
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Ankara first proposed the idea of U.S. special -operations forces accompanying Turkish troops late last month as it planned a joint mission into Syria's northern city of Jarabulus.

One can reasonably assume that the planned presence of U.S. forces amid a Turkish invasion has the sole purpose of deterring Russian or Syrian moves against it. With U.S. forces around the Russian command will have to think twice before bombing any Turkish advance beyond the borders of  their agreement with the Russians.

The deployment of some 40 U.S. special forces to Al Ra'i did not go well. The Turkish "Free Syrian Army" proxies threatened to kill the U.S. forces. They called them "unbelievers" and "crusader pigs" and the U.S. forces had to retreat under Turkish cover (video). Some FSA spokesperson later claimed that the dispute was over U.S. support for the Kurdish dominated SDF, which at times had fought against the FSA. Unconfirmed reports now say that the special forces are back in Al Ra'i after certain FSA groups were ordered out of the area. There are also reports claiming the U.S., after the special forces were chased out of town, "accidentally" bombed some FSA group in Al Ra'i. Ooops.