I think Canada is also a great ally for the States. That unprotected border is a huge advantage to their domestic defence. If you ever played Risk you might have an idea of how difficult Canada would be to control or move across to get to our neigbours to the south. Geography, weather, and size makes Canada difficult to occupy with any speed. That time factor makes America much more secure.
Also our continued relationship with the British Crown and service to Europe during the last two World wars probably would quickly draw most of the western world into such a conflict. That might be true in any case because of NATO but, again the fight wouldn't be starting on US soil.
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