I hesitate to make 1938-1939 comparisons because the world situation is far different now than then especially because of globalization. I do however agree with Kasparov the longer Putin gets away with this the more emboldened both the Kremlin and his troops will be. The problem here is two fold: The US is war weary after 10 years of doing Iraq and Afghanistan and Obama going to the country to put boots on the ground is a dicey decision and economically now isn't a great time to do that.
However, I think there are some deterrents you could do:
If I were President Cowan I would have already moved 30K troops into German for a little german vacation. I would also move some assets into place in the Gulf of Alaska. You know just routine troop movements and it would look very threatening and it might make Putin start thinking twice. The best way to get an enemy to stop doing something is to get them worried because then their own fear will either make them do something stupid that you can exploit or they will cease and desist. I might also start an economic war by asking the FED to dump a few billion Rubles into the world economy just to see what happens. Frankly, its time to get Machiavellian.
The problem here is two fold
Do you really think that if your two problems were absent -- if the US were not war-weary and if the economy were doing great -- then the US would be ready to put boots on the ground in Russia?
To quote a fellow named Vizzini, You fell victim to one of the classic blunders - The most famous of which is "never get involved in a land war in Asia" X-D
and it would look very threatening
I think it would look pretty silly and would give Putin a mighty political gift. He'd point a finger at your troops and yell that the...
Some of the comments on the link by James_Miller exactly six months ago provided very specific estimates of how the events might turn out:
James_Miller:
Me:
"Russians intervening militarily" could be anything from posturing to weapon shipments to a surgical strike to a Czechoslovakia-style tank-roll or Afghanistan invasion. My guess that the odds of the latter is below 5%.
A bet between James_Miller and solipsist:
I will bet you $20 U.S. (mine) vs $100 (yours) that Russian tanks will be involved in combat in the Ukraine within 60 days. So in 60 days I will pay you $20 if I lose the bet, but you pay me $100 if I win.
While it is hard to do any meaningful calibration based on a single event, there must be lessons to learn from it. Given that Russian armored columns are said to capture key Ukrainian towns today, the first part of James_Miller's prediction has come true, even if it took 3 times longer than he estimated.
Note that even the most pessimistic person in that conversation (James) was probably too optimistic. My estimate of 5% appears way too low in retrospect, and I would probably bump it to 50% for a similar event in the future.
Now, given that the first prediction came true, how would one reevaluate the odds of the two further escalations he listed? I still feel that there is no way there will be a "conventional battle" between Russia and NATO, but having just been proven wrong makes me doubt my assumptions. If anything, maybe I should give more weight to what James_Miller (or at least Dan Carlin) has to say on the issue. And if I had any skin in the game, I would probably be even more cautious.