[Focault's Pendulum]

Russian officials have been making noises about Estonia, due to its large Russian speaking population There might be a hesitation to this as Estonia is a NATO member. (2012)

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Residents of Estonia by ethnicity.

Estonians   68.7%
Russians   24.8%
Ukrainians   2.0%
Belarusians   1.1% Finns   0.8%
Others   1.6%

[FreeReign]

Residents of Estonia by ethnicity. Estonians   68.7% Russians   24.8% Ukrainians   2.0% Belarusians   1.1% Finns   0.8% Others   1.6%

Up against the Russian border, Ida-Viru county in Estonia is about 73% ethnic Russian. The other 14 counties in Estonia are largely Estonian.

Putin could probably walk in and take Ida-Viru but that might be his last play. No other region in Eastern Europe outside of Russia has a majority ethnic Russians.

Any other region besides Ida-Viru in Estonia(73% Russian) and Crimea in Ukraine(57% Russian) would be an entirely different circumstance.