We could easily deploy a few thousand troops into one or two positions that Russia would no doubt simply ignore. I cannot imagine them engaging in actual combat unless they are going to have reasonable expectations of support.
Given the current mindset of our current leadership, the Russians can easily count on no such thing ever happening, which reduces our most likely option to be bluster and saber rattling.
Which Obama seems to be willing to provide, exchanging showboating for domestic consumption for our remaining prestige and influence on the real affairs of the world.
"Right, as the world goes, is only in question between equals in power, while the strong do what they can and the weak suffer what they must.” ― Thucydides
I don’t think we can clandestinely place troops from Germany into position in the Ukraine. Any movement on their part would be known.
That’s why a drop by air strikes me as the best way to put a cannon fodder trip wire in Putin’s way.
In any case, I don’t expect the US to do it. However, on a different note I’m not real sure Putin and Obama aren’t working in concert.