Dirty bombs are a pretty empty threat and are a terrible first strike device. They don't "wipe out" anything. They are mostly area denial plan. Nobody would die

Dirty bombs are a pretty empty threat and are a terrible first strike device. They don't "wipe out" anything. They are mostly area denial plan. Nobody would die immediately. The US would be able to fully retaliate with regular nuclear weapons. They wouldn't even need to do that. The USA could defeat the Canadian armed forces in several weeks and then ethnically cleanse the country to make room for all the refugees from the dirty bombed cities. You'd also wouldn't be able to coat the entire seaboard with enough radiation to be all that dangerous. If Canada really wanted a nuclear deterrent, it could just build or buy a nuke. Hell the USA would probably give them nukes with the codes. It's done so with other NATO allies in the past.