The U.S. declined to provide a security "guarantee" because that would have amounted to a backdoor accession to NATO, and the U.S. could not grant that unilaterally. (An example of the U.S. honoring a prior commitment, by the way.) A "guarantee" would have required the U.S. to commit its military forces to the battlefield if Ukraine was invaded.
The U.S. instead provided a security "assurance" which was understood to mean a level of support short of U.S. troops on the ground. And in fact that is what the U.S. has been providing: intelligence and material, but no U.S. forces.
In other words, the level of support has been commensurate with the agreement signed. Until today, apparently.
> Clinton and Yeltsin did promise Ukraine “full guarantees of security, as a sign of friendship and good neighborliness.” The two leaders also reaffirmed “the obligation to refrain from the threat or use of force against the territorial integrity or political independence of any state,” including Ukraine.
Also, that was not any form of an agreement, it was just words in a meeting. Nobody signed, it was not ratified with US congress as a treaty.
If you take that as binding then all the verbal commitments made to Russia that NATO won't expand eastwards should also have been binding, and they were broken first. I know you will get angry that I mention it, but it's a fact.
Well, I did say not legally binding. But you can see how someone might interpret "Russia and the U.S. will give full guarantees of security" to mean "Russia and the U.S. will give full guarantees of security" if they weren't really paying attention?
It was not even an agreement at all, nevermind binding. It was words said by the Russian president. Not a contract, not a memorandum, just words in a meeting. If Ukraine took one sentence said by the Russian president in a meeting as security guarantees from US (emphasis on the word guarantees) then they are the problem.
Security guarantees would at minimum require a treaty ratified by congress in US, something which I don't think is on the table any more at all.