Canada does not, but if the US wanted to unilaterally attack and occupy our land, there’s very good reason to believe we would look closely at all offers to move nukes from the EU, China, or Russia into Canada, and even potentially use them. We burned down the white house once - next time it might need to be the entirety of DC…
I hope the NATO push would be stronger than simple arms supplies, if nothing less based on the likely crumbling of the alliance. If it were to fall it’s very conceivable Canada would strike deals with the Chinese, or even Russia to maintain its independence. Protected guerrilla warfare regardless, which the USA does not have a great track record, especially considering it’s never tried it with as well armed and aligned nation as Canada.
It would be an invasion and occupation. Very very few Canadians would cooperate with the annexation, and more than likely it would look something more like Afghanistan or Ukraine, but with much better equipment being provided to resistance forces.
I would hope our NATO allies would fulfill their commitments too!
The only way Canada would join the union would be as an occupied territory, and we would do everything possible to fuck shit up from within.
A 500t nuke could take out the white house, capitol hill and not too much fallout beyond.