The US would tear itself apart if it attacked Canada any time in the next 50 years minimum. They are in an essentially M.A.D. situation with the US even without nukes because American citizens like Canadians and view them as brothers on top of Canada having tons of European ties and military treaties. Wasting money on nuclear weapons isn't going to put them in any better position than they are already in. The only effective strategy if the US started shit would be to bunker down and go full insurgency against occupation, throw out a bit of good pr/propaganda, and wait for internal US strife to collapse the nation as the US is left completely isolated from the entire world with enemies on all sides and within.