Prof. Jeffrey Sachs of Columbia University gave an interview to Tucker Carlson recently, in which he stated (25:45 into the interview):
The US had promised, unequivocally, to the Soviet Union, in the context of German unification, as of February 1990, that NATO would not move one inch eastward. This remains, by the way, highly contested to this day. But if anyone wants the information, you go on something called the National Security Archive of George Washington University, and you can read the dozens and dozens of statements and all of the archival material making completely absolutely unequivocally clear that the United States and Germany promised that NATO would not move one inch eastward. So the record is absolutely clear.
But this promise was not part of any ratified treaty, and the Soviets must have known that. Would it be reasonable to argue then that such a promise was, in fact, not made in a way that formed a binding obligation in international law?