BFD.
My grand-uncle was a captain in the Mexican Punitive Expedition of 1916. And now, it's long past time for a do-over.
The entire history of humanity is armies crossing borders.
If Ukraine is strong enough, or has strong enough friends, it won't be a problem. If not, not.
Either way, none of our business.
"Europe" dragged us in twice and the playing cards are shaping up the same again. This time with lots and lots of technology that kills humans dead from a very long distance.