Hi
@Juan,
My understanding is the U.S. (and I imagine other European countries) first assured Mikhail Gorbachev, Russia (and I believe the old Soviet Union) in 1990 under the George Bush Sr. administration that NATO would not expand eastward if the Soviet Union would not stand in the way of West and East Germany unification. Other assurances were given at various times after that as well, with some I assume coming from the Clinton administration, and others coming from various West European powers.
So is the invasion of Ukraine solely the result of the U.S. and/or NATO reneging on their assurances that NATO would not expand eastward? Hardly, in my opinion. The real reason for the war in Ukraine is because of the delusional ambitions of one man by the name of Vladamir Putin, who fancies himself to be among the great Russian leaders of all time.
And what did all these leaders such as Joseph Stalin, Peter the great, Catherine the great, Ivan the terrible, etc., all have in common that he has tried to emulate? They were all expansionist powers who worked diligently and ruthlessly on expanding the Russian empire beyond its own borders. This meant the brutal subjugation of its neighbors such as Latvia, Lithuania, Estonia, Poland, Ukraine, Finland, Hungary, and on and on.
When these countries found themselves free after the breakup of the Soviet Union, they very actively pursued membership into NATO with a zeal that could only be understood by someone who'd been under the yoke of Russia for centuries. So yeah, the U.S. and Europe in allowing these countries to join NATO did break their assurances to Russia that NATO would not expand eastward. But I think it's important to understand the context of how this happened. These countries wanted protection from almost inevitable future Russian aggression! And the best assurance of that was through membership in NATO.
I don't think it can be repeated often enough that Russia has for many centuries been an expansionist power intent on expanding its borders and its empire. Vladamir Putin realized that Russia without Ukraine could never be an empire again. His proposition that Ukraine being part of NATO was somehow a future threat to Russia and needed to be neutralized is pure propaganda, and a total red herring. From just about any objective viewpoint, NATO was ALWAYS a DEFENSIVE organization, and never once posed a threat to the Soviet Union or Russia. Unless you consider it a threat to not allowing Russia to once again dominate its neighbors, just as it had done for centuries.
BTW, just for context. Russia has lied and broken its own assurances about many, many things over a long period of time. It's how they operate. Putin has assured the West and the Russian people that he would uphold the traditions of democracy in Russia, and allow basic civil liberties and freedoms for all. That's hardly the case, and Ukraine has felt the threat of his autocracy becoming ever more and more dangerous. It's why they sought NATO membership, and entry into the EU. Ukraine becoming free from his grasp was something Putin just could not fathom or countenance. Ukranians wanted to assure their freedom, Putin wanted their subjugation.