Apart from Estonia and Latvia -- where ethnic Russians constitute over a quarter of the population, but where NATO membership raises the risk for the Kremlin -- by far the most likely target is Ukraine. Kiev has repeatedly defied and angered Russia by the domestic politics of democratization, a decidedly pro-Western orientation, and the eagerness of its leadership to join NATO. Nearly one in five Ukraine citizens are ethnically Russian (a total of almost eight million) and live mostly in the country's northeast, adjacent to the Russian border.
Russia might start by annexing Sevastopol, a city of 340,000 on the Black Sea. Alternately:
A potentially bolder (and likely bloodier) scenario might involve a provocation by the Moscow-funded, and perhaps armed, Russian nationalists (or the Russian special forces, spetznaz, posing as irredentists). They could declare Russian sovereignty over a smaller city (Alushta, Evpatoria, Anapa) or a stretch of inland territory. In response, Ukrainian armed forces based in the Crimea outside Sevastopol would likely counterattack. The ensuing bloodshed would provide Moscow with the interventionist excuse of protecting its compatriots -- this time, unlike in South Ossetia, ethnic Russians.Since Russia is a dictatorship, albeit with nominal 'elections,' and Ukraine is a democracy, one can expect that the Democrats will instinctively side with Russia.