Ukraine gained it's independence in 1991. The US didn't even have military units stationed permanently in Poland or the Baltics until now.
Cuba has had Soviet and/or Russian military on it since Castro came to power 60 years ago. Venezuela also has a Russian presence there.
One of the ironies of history is that the Soviet's and Russia's allies seem to ruin their own countries far more effectively than any US invasion could.
What you are implicitly arguing for is a return to great power "spheres of influence" which might have made sense in the 19th century era of world spanning European empires. But running foreign policy on the basis of great power spheres of influence is what got us WWI and WWII, which is why the US has rejected it as a doctrine since 1945.
NATO was never just about protecting Western Europe: it was meant from the beginning to offer hope of democracy to the "captive nations" in Eastern Europe. That's the real threat Ukraine's integration with NATO and the EU offers to Putin: a culturally close Slavic neighbor that turns into an economically and politically successful democracy. Its very existence encourages Russians to do the same. I was there in 2016. It struck me as being like Poland was 20 years before.
Not true.
Our hands aren't clean here. We supported the coup of Ukraine's democratically-elected leader in 2014 because he was friendly to Russia and we've been provoking Russia with our NATO expansion and our military involvement with Ukraine.
Our government does this because our government does whatever it wants.
(And it gets away with it because millions of Americans simply refuse to believe that our government can do anything wrong.)