Not to worry. The Budapest Memorandum assured Ukraine that getting rid of their nuclear weapons (which were under Russian operations control) was the best move and the US and Brittan would have their backs in the event of trouble. It was agreed to by the Russians too so no need to worry /sarcasm
But the Budapest Memo was not a “treaty” (according to the Russians, which is correct). Much like the “promise” that NATO would not expand closer to Russian border, which the Russians claim was an ironclad super-treaty.
Let's apply some logic here. Split Ukraine in half. If any surrounding country wants the western half, let them have it. Just keep the Kiev coup gov out of the eastern half, where they aren't welcome. If people aren't happy with the half they're in, facilitate a move.
Am I missing anything here? I just can't imagine how it doesn't escalate to a major war if US trained, armed, and advised troops show up in eastern Ukraine.