While American military personnel have been operating in Syria for some time, they've consisted mainly of Special Forces units and advisors (which means about as much as it has every other time "advisors" have been sent abroad). Sending in hundreds of Marines in the span of days is a fast and significant escalation with a mission that has no quick end in sight. The Marines aren't just there to support local units fighting IS, they're also there to stand between factions that are fighting each other while fighting IS (which pretty much sums up the state of Syria right now). And while the Marines are there mainly to support local fighters, military missions have a tendency to expand out further.
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