Georgia has asked for US military airlift help to transfer its troops in Iraq back home to Georgia.
If Putin pushes to intercept any such troop convoy with force we would see war break out between US and Russian military forces. The end game here
is nuclear exchange.
I don't know if we would send any MAC transports' in unescorted; However, If we DO send them, with fighter cover, and it turns' into an all-out battle for air supremacy over Georgia-Ossetia, then we might see further U.S. involvment.
As far as going Nuclear, well...I don't myself think immediately. The Russian's aren't fighting on their own soil, technically; Even if they pull back and lose Ossetia to the Georgian's, They don't lose anything they didn't already have. At the same time, we are helping an ally, BUT, not a full-fledged NATO member. If this was a full NATO country, my guess is that do to treaty obligations, we'd already have our own forces' involved. But I don't think we have any such obligations' at the current time. I'm looking to see some kind of diplomatic solution fairly shortly.