No Western power in modern times has put its troops inside the borders of a sovereign nation without being invited either by the nation itself or on behalf of an international coalition cooperating for what they believe to be the common good.
USA. Syria. Literally still happening. Your statement is still technically correct, if you're willing to go mask off. Feel free.
So we’re going with the approach of arming & training Ukraine’s military to hold the Russians to a standstill and maybe even push them back even though they are heavily outnumbered.
I'm not sure why you need "even though" there. Yes, Russian forces in country 404 are heavily outnumbered.