The obvious strategic move is to decapitate Ukraine and install a puppet. — RogueAI

Exactly. — ssu

It's not obvious at all for anyone with more than a surface-level understanding of the political reality in Ukraine.

You can't just "decapitate" a country that has been preparing for war for a decade with western backing. And you don't just "install a puppet" for the same reason.

A puppet regime would probably last about a day, and all you would have achieved is to give the foreign backers total control over the country.