The entire point of NATO and other treaties is that every signatory is willing to risk their country to protect the other signatories. It makes the game theory choice to attack stupid.
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.
The us military is also too professional to consider following orders to attack Canada. They would not accept the risk and reputational damage that would come with breaking the NATO treaties and undermining the general security of the west.
Armed forces are friendly to who they are being ordered to be friendly. And they don't have a an issue with any other armed forces unless being ordered to have an issue.
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.