For the States, united, to make a binding promise to a foreign country requires consent from the states in the form of 2/3 of their delegates (the Senators) to ratify something called a treaty.
In the case of Ukraine there is a piece of paper signed by Bill Clinton, which has no binding effect on the 50 states which would have to provide the men and the money to fight a war.
If we had ignored Ukraine after the collapse of the Soviet Union. Ukraine was more likely to keep their nukes.
Russia had the codes, but if Ukraine really needed the codes, they would find a way to get them.
Russia may get angry at Ukraine for not giving up their nukes. But what will they do about it?
Just look at North Korea. Everyone leaves them alone.