You see, this here is exactly where isolationist thinking goes wrong, because it only thinks in the 'right now.'
Does Russia taking control of Ukraine threaten the US?
Only a little bit, and that's economically. It's a much bigger threat to Europe, but again, that's not a direct threat to the US.
What about when Poland is next?
No, it's only a modest economic threat, no major threat overall. Poland is even less important to the US economy.
Moldova? Romania? Latvia? Estonia? Lithuania?
None of those are that big a deal to America either, economically or militarily, but at this point, Russia has just about finished reconsolidating the Soviet Union.
And then Germany is next.