I wouldn't trust the US either, although having economic leverage helps. But are you referring to the Budapest memorandum with
They guaranteed Ukraine's integrity in return for denuclearization
If yes I don't understand how this misunderstanding still has to be corrected. Read it for yourself here (swapped link to something working, pdf warning)
At no point does it promise defense against foreign third-party attacks except in case nuclear weapons get involved. Every security assurance only covers the nation themselves. Something Russia has obviously broken (but there are no penalties stipulated), whereas the UK and USA have held up their part of the deal. Unless they have invaded Ukraine themselves without my knowledge.___