[nl]

Russia _hates_ that Estonia, Lithuania, and Latvia are all part of NATO.

But when they were accepted Putin didn't feel strong enough to do anything about it.

Russia has been advancing it's "sphere of influence" political theory, whereby independent countries with that "sphere of influence" don't get the right to make their own alliances. That "sphere of influence" includes all Eastern European countries that border on Russia at least.

It hasn't been widely reported on, but the initial demand that led to this war was that NATO withdraw all infrastructure install in post 1997 expansions:

Russia published two lists of demands — for Washington and for NATO — the latter calling for the removal of all NATO military infrastructure installed in Eastern European countries after 1997, effectively attempting to rework the consequences of the Soviet Union’s collapse in 1991, which left Russia weakened for years.

https://www.washingtonpost.com/world/2021/12/17/ukraine-russ...