Hmm, I think another reason for this to happen now, is that the border between Belorussia and the Ukraine is covered by the Pripet Marches, that are neary impas

Hmm, I think another reason for this to happen now, is that the border between Belorussia and the Ukraine is covered by the Pripet Marches, that are neary impassible by heavy equipment when not frozen. Without access that way, Russia would have to fight their way from the east, and forced to cross the Dneipr river, which forms a very significant obstacle. This could delay an invasion by a month or more.