The controversy regarding the legitimacy of eastward NATO expansion relates to the aftermath of the Revolutions of 1989, when the fall of Soviet-allied communist states to opposition parties brought European spheres of influence into question.
Russian authorities claim that agreement on non-expansion of NATO to Eastern Europe took place orally and the alliance violated it with its expansion while the leaders of the alliance claim that no such promise was made and that such a decision could only be made in writing.
Soviet President Mikhail Gorbachev, who participated in the 1990 negotiations, subsequently spoke out about the existence of a "guarantee of non-expansion of NATO to the east" inconsistently, confirming its existence in some interviews and refuting in others.
Among academic researchers, opinions on the existence or absence of a non-extension agreement also differ.~ https://en.wikipedia.org/wiki/Controversy_regarding_the_legi...
OTOH, I understand there was an actual drawn up agreement regarding Ukraine giving up post Soviet nuclear weapons, Russia agreeing to Ukranian autonomy and the US agreeing to provide secuirty for Ukraine.