Both the US and Russia have had troops/SF in Syria for more than a year now. In fact, we have actually backed two sets of rebels in conflict with each other. We're arguing Russia is backing Assad. Russia argues they're attacking ISIS (among like five other groups).
I think the reason everyone is so unsettled is that usually when we talk about the government bombing someone, we anticipate sound military planning, a concrete objective, direct cause and effect and a reasonable understanding of the purpose. Take the Libyan raid in '86:
1) Libyan terrorists blow up a disco in Berlin with US servicemen in it
2) US sends four dozen aircraft to bomb Gaddhafi's camp
Or shit, the invasion of Grenada. Or Panama. Or, better model, the Serbian War. But everybody who so much as watches the Today show knows that
1) Russia is in Syria
2) Russia is fighting ISIS
3) We are fighting ISIS
4) Russia is backing Assad
5) We are condemning Assad
6) Please go to commercial my head hurts
So nobody knows how this plays out. But the fact that we bombed an airfield that the Russians had under construction already (but weren't currently flying out of) leads me to believe that this is some good ol' fashioned ol' Reagan-style saber-rattling.