In a related question on why Russia is against Ukraine joining the EU, @user4012 writes the following:
Basically, Russia has no natural defensive perimeter of its core. Thus, its permanent strategy is to surround itself with satellite states which provide defense in depth and natural defensive perimeters of their own. This is elaborated extensively on in Statfor writeups on Russia, if you want a more detailed look.
If Ukraine is integrated into the West, Russia sees it as a strategic military threat - there's very little defense should Ukraine be a base of attack as opposed to a defensive buffer.
But why would Russia fear a land-based invasion from NATO in the first place? It has plenty of nuclear warheads and ICBMs to completely annihilate most of the NATO members capitals. Likewise the core NATO members all have nuclear weapons so they would know better than to risk a nuclear war against Russia.
So to me it seems like the US could have its entire army at a base in Finland and Russia could still sleep in peace. What is wrong with my assumptions?