With today's America, the answer is very clearly that it will do whatever it thinks is in its short term interest, or really in those of Trump and his biggest backers. Guarantees are worth nothing, would trust China more because at least they have a long term view. In fact, it has been a much better ally to Russia than the US has been to Europe or Ukraine.
More nations will want nuclear weapons, more might turn to China, the world will get less secure and more volatile.