Even if Russia had the ability to project such a force, in all probability, all that might do is start the lead up to WWIII.
Obama would brush aside such a bluff.
We put 100K GI's in West Germany with the assumption that the massed Soviet armies would swat them aside like flies if the balloon went up. They were a tripwire force, a sacrificial lamb. What that guaranteed was not American victory against Soviet conventional forces, but American intervention that could run the spectrum from writing off the force to launching a nuclear first strike. The Soviets never crossed that line because it wasn't clear whether we would actually go all the way, and burn the Warsaw Pact's thousand largest cities to the ground in a matter of hours.
When Yeltsin landed paratroopers at Pristina, Clinton backed off because he made the same calculation as the Soviets did during the Cold War. Thanks to a decade of strong commodity pricing, the Russian nuclear arsenal has been revived. Putin may feel that he needs to re-establish Russia's credibility. If he decides to make a stand over Syria, Obama will have to decide whether to back down or risk a nuclear confrontation. It's clear, from the way we've avoided bombing North Korea, that our leadership takes North Korea's threats very seriously. It would be risky to assume that no Russian leader will ever up the ante. Syria is a former Soviet client state. I'd say he's within his rights to assert a Russian interest in keeping a traditional ally in power, just as we're within our rights to prevent Russia from overthrowing the government of one of our traditional allies.