At the time of German reunification, NATO promised Gorbachev that it would not expand to the East in the case the USSR gave up the Warsaw Pact and allowed the fall of Berlin wall.
Recurring pro-Kremlin disinformation that NATO had promised that the Alliance would not expand to the East.
NATO Allies take decisions by consensus and these are recorded. There is no record of any such decision having been taken by NATO. Personal assurances from individual leaders cannot replace Alliance consensus and do not constitute formal NATO agreement.
In addition, NATO's "Open Door Policy" is based on Article 10 of the Alliance's founding document, the North Atlantic Treaty (1949). The Treaty states that NATO membership is open to any "European state in a position to further the principles of this Treaty and to contribute to the security of the North Atlantic area".
Moreover, at the time of the alleged promise, the Warsaw Pact still existed. Its members did not agree on its dissolution until 1991. The idea of their accession to NATO was not on the agenda in 1989. This was confirmed by Mikhail Gorbachev himself in an interview with Russia Beyond the Headlines:
"The topic of 'NATO expansion' was not discussed at all, and it wasn't brought up in those years. I say this with full responsibility. 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".
The Russian narrative that NATO, a defensive alliance, is de facto aggressive is not new. Read more in this 2015 Tagespigel analysis.
See more disinformation cases on NATO.