LOL.[1]
The only preventative measures which would've worked would be diplomatic and economic counter-pressure from the UN and more specifically Europe and other NATO allies, or substantial military build up capable of seriously endangering US conventional forces.
Ukraine would be in a much different military position today if it were a pariah state, as opposed to a nation declaring a strong intent to join both the EU and NATO. Turns out diplomacy, allies and soft-power are actually kind of useful things when you're not the dominant military hegemon.