Graham Allison consider it a remote possibility, but not as remote as he wishes it were. Excerpts:

The rapid slide from lawlessness to violence that has claimed the lives of more than sixty people in the Ukrainian cities of Donetsk, Slovyansk, and Odessa in the past week sounds alarms that should be heard more clearly in Western capitals. The strategy Washington and the Europeans have chosen that focuses on the villainization of Putin (much as he deserves it), calls on him to withdraw support for the separatists, and threatens further sanctions if he does not is bound to fail. It will not stop the killing. It will not prevent the de facto dismemberment of Ukraine. It will not deter Putin from continuing whatever role he and Russia are playing in this process. And it fails to address the risk that what happens in Ukraine does not end in Ukraine.

What happens if the 25 percent of Latvians who are ethnic Russians move to take over government buildings, as pro-Russian Ukrainian separatists have done? Allison says that the Latvian government would have no choice but to attack them … but that would put Putin on the spot regarding his pledge to defend ethnic Russians. And if Putin dared to send troops into Latvia?

The brute fact that Latvia is a member of the NATO alliance is hard to ignore. The United States and other members have solemnly pledged themselves to regard “an attack upon one as an attack upon all.” But will German troops come to Latvia’s rescue? And if they did, would a majority of Germans support that action? Would the French, or British? Would Americans?

If we do, we will cross a bright redline Republican and Democratic presidents assiduously avoided over four decades of Cold War: American and Russian troops would be killing each other. Any such conflict would raise risks of escalation in which each nuclear superpower remains capable of erasing the other from the map. But if we don’t, we will see a precipitous collapse of the credibility of U.S. security guarantees that have been the central pillar of the international security architecture the United States has constructed since World War II. Not only European allies, but Japan, South Korea, and others who have staked their survival on a U.S. security umbrella will look to their own defense.

Would you support going to war with Russia — which still has nuclear weapons, recall — over Latvia? That’s hard to support. But if America did not honor its NATO commitments, the entire world would become a much more dangerous place, as Allison avers.

This is why it’s not unthinkable that the Ukraine crisis could spiral out of control.

Read the whole thing. To avoid the worst-case scenario, you have to confront the worst-case scenario.