Poland would also respond if Russia threatened the rest of Ukraine beyond the Donbass and Crimea. They would move into Western Ukraine.
Part of me would love to see Ukraine absorbed into Russia, just to teach them and the rest of the world a lesson about mindlessly acting as a tool of the collective West and the military industrial complex, but, alas, the sheer amount of money that would be required to get the entirety of Ukraine up to Russian standards just in terms of physical infrastructure would be so massive as to be absolutely unimaginable.
“ They would move into Western Ukraine.”
No they wouldn’t, but partition of Ukraine along the old Austria/Russia border is inevitable.
Poland can get in the game and restore Kresy, or can get run over.