But it’s all academic anyways. The US military would eagerly lead a coup before willingly attacking the UK or Canada, particularly if non-conventional means are threatened.

Consider that nearly every officer has done several war games or exercises alongside UK or Canadian officers. This is true both of NCOs and senior leadership. Being friendly with their armed forces is institutional. It would be easier to convince the Marines to attack the Army than to convince them to attack their Canadian counterparts.

In this hypothetical situation the opinions that matter are those of the men in the subs that have just seen their family and friends wiped out.

They've got roughly 120 active warheads to play with and realistically no one is going to stop them all.

So if you add up the damage from that plus the loss of a major trading partner and military ally it kind of seems unlikely to be a profitable venture for anyone involved.