as a response to libya, russia has invested heavily in syrian anti-aircraft weaponry. the americans rely on airpower; the russians & syrians have neutralized it (not in terms of absolute power, but in terms of ability to cause serious casualties and bad messaging). that's why they're funding the rebels. if it was as easy as airstrikes, assad would already be dead.

they are going to eventually get some strikes in to demolish the infrastructure (it sounds barbaric because it is barbaric, but they're going to want to eliminate the ability to recreate a sovereign state), but first they need a force on the ground that will take out those anti-aircraft systems - and ensure there aren't russian backdoors in the software.

they may actually give up before they get there. the americans really played dirty launching a distraction in ukraine. let's see what the russians have in terms of distracting abilities...