Lots of talk among commentators about promises made by the West in order to have the Soviet Union disbanded
... in like 1990 (but over several years, and under several US presidents making having talks). The idea was that in the talks, there was nothing in writing, but it was made clear to Moscow that if the SU allowed the reunification of Germany that NATO would not be expanded. That NATO troops would not be able to enter into the eastern part of Germany (only to West Germany).
It wasn't a treaty, but it was (people are saying) the agreement that was made.