It was made during discussions about the reunification of Germany, by John Major as Foreign Secretary and James Baker as US Secretary of State. Unfortunately for the Russians it was made "informally", not written down or recorded in any subsequent aide-memoire, let alone treaty, and therefore neither the British nor the American government consider themselves bound by it.
I put down a question in the House of Lords about this, after Russian officials had begun complaining about it - including Mr Gorbachev in London last year - and the answer came from the Foreign Office that there was no "formal undertaking". There was no denial of the informal undertaking, of which the Russians have apparently produced a rather confused tape.
That the Russians should have thought an Englishman's - or an American's - word might still be his bond showed naivety and a lack of diplomatic professionalism. All the same, there is a sense of betrayal, and it is no wonder they are not too keen now on any simple "political charter" with Nato which might turn out no more binding than this other "political" undertaking.
House of Lords