Canada, and basically any other non-nuke country bordering a country with nuclear power plants.
Which, I don’t exactly know Canada-US defense agreements, but given we share assets like NORAD—I assume an attack on southern Ontario would be tantamount to declaring war on the US, which would probably go poorly for everyone.
EDIT: Oh, strike that, they literally mean Canada attacking the US, which is the stupidest idea I’ve heard in a while.