The expansion of NATO deep into former Warsaw Pact countries.
That's what you're replying to. Several former Warsaw Pact countries joined NATO. The West didn't invade them.
Every empire needs aligned, friendly, or at least, neutral buffer states on their borders - and will violently defend incursions into their buffer zones. Observe the shitshow over Cuba - sixty years of sanctions, terrorism, assassination attempts, an invasion, and a nuclear crisis later, the United States still hasn't forgiven it.
The United States was ready to bathe the entire world in nuclear fire - because a neighbouring country entered into a military alliance with the USSR. Let that sink in... And then consider what expanding NATO into Eastern Europe, US support of Georgia, regime change in Syria looks like from the Russian point of view.
Meanwhile, in the present, 21st century, the US is busy building a missile defense shield in Poland. [1] (While claiming that it is built to defend against, of all places, Iran. [2] Seriously?) It is intended to neuter Russian nuclear capability... Meanwhile, Minutemen ICBMs continue to be able to strike Russia.
I ask again - how would the US react if China, or Russia started building army bases, and anti-ICBM installations in Canada, Mexico, and Cuba? How would the American empire respond to a political incursion into its buffer states?
[0] http://www.ww.nuclearfiles.org/menu/key-issues/nuclear-weapo...
[1] https://en.wikipedia.org/wiki/Aegis_Ballistic_Missile_Defens...
[2] https://en.wikipedia.org/wiki/United_States_missile_defense_...
But if other countries, especially smaller ones, want something, then to hell with them!