For those who want a nuanced take, rather than a kneejerk, totally partisan overreaction, to what happened in the 1990s when NATO expanded, I found this to be a very informative article.
TL/DR version? As usual, instead of being the crystal clear black and white answers today’s right seems to yearn for, what happened was a lot more nuanced than “NATO broke a promise”.
In short…
The US thought it won the Cold War and wanted to order European security the way it wanted to do so…the mistake Bill Clinton made was thinking he could convince Boris Yeltsin that this wasn’t a threat to Russia. He put off expansion long enough for Yeltsin to win another term in office, but then the Expansion Now faction won out.
Russia thought it defeated communism and the end of the Cold War was a mutually agreed “cease fire” and wanted to be a partner in building European security…and not a junior one. But once those in the West who favored expansion got the ball rolling, Yeltsin was powerless to stop it…but he made his opinion known many times…especially after Kosovo.
The Central European countries quite obviously were hungering for the umbrella of NATO protection, having had a long history of being overrun and used as pawns in Great Power Politics.
Neither the West nor Russia could find a place for Russia in the new order of Europe, despite many tries…with Partnership for Peace, with Russia/NATO cooperation, the NATO/Russia Founding Act…and so Russia began to see the 1990s as a Great Humiliation.
Thus, as usually happens, Europe slid almost inexorably to where we are today…with Russia and the West again as adversaries, in a far more unstable Cold War than existed in the era of 1945-1991…where neither side has the power and influence they had in the past to create a true stalemate…where both sides now are far more entwined with each other than they were in the postwar period.
You can play the blame game…or you can try to learn and understand.
The US, the EU, Russia- none are totally blameless in this matter. But neither are any of them totally to blame.