> The US promised NATO wouldn't expand east
No, it didn't, at least in any way which anyone who is qualified for even an entry-level position in any diplomatic service on Earth would mistake for a binding commitment even on the then-current administration, much less indefinitely into the future. (The most commonly cited statement on this is actually a reference to deployment of troops within United Germany in the near term post-unification, and the second most commonly cited does refer to NATO expansion but is a communication of items that could be on the table for discussion if Russia was willing to come to the table on German unification, not a commitment; but there also some claims that some other private communication not reduced to a binding international agreement occurred.)
If it had made such a promise to the USSR in a binding way, we could discuss, based on the specific form of the agreement and the subsequent diplomatic action relevant to it after the fall of the USSR whether it was still binding between any parties and which nation or nations were the USSR’s successors in interest and what were the legal effects were of any potential disunity between those parties.
But since it didn't, the whole set of questions is moot.
> and then immediately reneged on it
NATO expansion to Eastern European nations beyond the former East Germany didn't occur until 1999, which is not “immediately” after an event alleged to have happened in 1990.
No, it didn't, at least in any way which anyone who is qualified for even an entry-level position in any diplomatic service on Earth would mistake for a binding commitment even on the then-current administration, much less indefinitely into the future. (The most commonly cited statement on this is actually a reference to deployment of troops within United Germany in the near term post-unification, and the second most commonly cited does refer to NATO expansion but is a communication of items that could be on the table for discussion if Russia was willing to come to the table on German unification, not a commitment; but there also some claims that some other private communication not reduced to a binding international agreement occurred.)
If it had made such a promise to the USSR in a binding way, we could discuss, based on the specific form of the agreement and the subsequent diplomatic action relevant to it after the fall of the USSR whether it was still binding between any parties and which nation or nations were the USSR’s successors in interest and what were the legal effects were of any potential disunity between those parties.
But since it didn't, the whole set of questions is moot.
> and then immediately reneged on it
NATO expansion to Eastern European nations beyond the former East Germany didn't occur until 1999, which is not “immediately” after an event alleged to have happened in 1990.