If Ukraine is annexed by Russia, the defense of the Baltics becomes much more difficult, as Poland, Slovakia, Hungary, and Romania then become susceptible to Russian attack. (Moldova too but they're not in NATO)

In the graph of adjacent nations, Ukraine has a high betweenness centrality

https://www.researchgate.net/figure/Ranking-of-centrality-va...

What's Ukraine bringing to the table that's not already accomplished by the multiple NATO members already bordering them?

If Ukraine becomes Russian, then Russia will border NATO members, too. (Because Ukraine borders NATO members.)

Putin wouldn't be rational if he thought NATO would attack. He would instead be dumb.

The things about NATO is what he says to mislead people -- and maybe that is a rational step in his restore-the-Soviet-Union dreams

The Baltic states already border Russia.

Russia is a tiny economy compared to NATO.

With modern logistics, I don't buy the geographic argument.

To be this is just Putin playing cards the rest of the civilized world have decided you can't play.