A battalion of US Marines could defend Ukraine and two battalions of US Army Rangers could push the Russians out. Hell, a company or two of Abrams could take out every armored vehicle Russia has sent into Ukraine. We could have ended this war years ago if we had done as we promised.
This isn't said enough, but it's often forgotten that in 1994's Budapest Memorandum, the US and the UK guaranteed they would support and defend Ukraine's territorial integrity in the event of any future Russian assault. Ukraine should have been incorporated into Nato at the time. And kept it's nukes
Joined NATO yes, nukes, not so much. If they did then have NATO make sure they were relatively safe and functional. The Soviet Union was a paper tiger and their shit barely worked.