The trouble is, OP, that Russia will not stop at Ukraine.
Russia justifies what it is doing by saying that Ukraine was historically part of Russia.
On this basis Russia may also help itself to Latvia, Estonian, Lithuania, Finland and parts of Poland, all of which are in the EU and were once part of the Russian Empire and/or Soviet Union.
Even Alaska might be next.
Trump has shown he’s only up for whatever the USA can get out of it and that he isn’t interested in defending Europe.
He’s even displayed Russia-like tendencies in saying he wants to grab Greenland and Canada.
Remember, even in WW1 and WW2 they only got involved (and thank goodness they did) when American interests themselves were threatened, not out of some benevolent desire to protect Europe from Hitler.
What we need to do is draw our true friends closer (the EU, Australia, Canada, New Zealand) and, regrettable though it is, up
our defence spending.
We can’t send troops to Ukraine ourselves because that really would cause WW3 but we need to give them the tools to finish the war because the USA can’t be relied on now, sadly.