What lies behind Moscow's willingness to crush Georgia with overwhelming force? Analysts have highlighted Russia's newfound economic confidence, its determination to undo its humiliation of the 1990s, and its grievances over Kosovo, U.S. missile-defense plans involving Poland and the Czech Republic, and the eastward expansion of NATO.

But there may be another major, overlooked element: Has a shift in the nuclear balance between the U.S. and Russia helped embolden the bear?

Under...