There are multiple memorandums and cable transcripts in the link I posted. Verbal guarantees were given on many occasions but conveneniently forgotten or dismissed later on.

I fail to see how smth like Bucharest memorandum is so different in terms of importance (in you eyes) provided it was not recognized as legally binding and was described as a "political commitment" by the US in the context of its sanctions on Belarus.

> Across history, the Russians were the only constant invaders.

Which history? It is just a fact of life that smaller countries are more frequently invaded and/or being controlled (not that it doesn't happen at all to bigger ones).