"In 2014, the former Soviet leader Mikhail Gorbachev marked the 25th anniversary of the fall of the Berlin wall by noting in an interview that Nato’s enlargement 'was not discussed at all' at the time:
'Not a single Eastern European country raised the issue, not even after the Warsaw Pact ceased to exist in 1991. Western leaders didn’t bring it up, either.'" [1]
Maybe Gorbachev feels embarrassed that he didn’t get any formal guarantees from the West.
In any case, declassified US, Soviet, German, British and French documents[1] show that multiple Western leaders assured Gorbachev himself that Soviet security concerns would not be threatened. And, of course, it was specifically Nato that was under discussion.
'Not a single Eastern European country raised the issue, not even after the Warsaw Pact ceased to exist in 1991. Western leaders didn’t bring it up, either.'" [1]
[1] https://theconversation.com/ukraine-the-history-behind-russi...