You may find the accounts from two Ambassadors presented in this write-up interesting: https://transatlanticrelations.org/wp-content/uploads/2019/1...

Here's the direct quote from Ambassador Braithwaite's letter. Braithwaite was UK’s Ambassador to the Soviet Union and Russia until 1992.

The assurances Russians point to the following:

Assurances given in 1990:

• James Baker, US Secretary of State, 9 February 1990: “We consider that the consultations and discussions in the framework of the 2+4 mechanism should give a guarantee that the reunification of Germany will not lead to the enlargement of NATO’s military organization to the East”;

• Helmut Kohl. German Chancellor, 10 February 1990: “We consider that NATO should not enlarge its sphere of activity.”

Assurances given in 1991:

• John Major. British Prime Minister, Speaking to Defence Minister Yazov, 5 March 1991: “He did not himself foresee circumstances now or in the future where East European countries would become members of NATO;

• Douglas Hurd, British Foreign Secretary, speaking to Foreign Minister Bessmertnykh, 26 March 1991: “[T]here were no plans in NATO to include the countries of Eastern and Central Europe in NATO in one form or another;”

• Francois Mitterrand, speaking to Mikhail Gorbachev, 6 May 1991: “Each of the [Eastern European] countries I have mentioned will seek to ensure its security by concluding separate agreements. With whom? With NATO, of course. ... I am convinced that is not the right way forward for Europe.” This was, of course, a prediction, not an assurance.

[Author’s note: recently declassified material published by the National Security Archive contains some additional references to similar “assurances.’]

This factual record has not been successfully challenged in the West. The remarks by Major and Hurd are confirmed by British records. I was present on both occasions.