This came up in a seminar I took last year on military history. The professor asked us, "What would you do if the Canadian army invaded?" and the answers ranged from "Throw a hell of a party" to "You could only tell by the tidied-up campgrounds and the thank-you notes."
After the failed invasion of Canada in 1812 the two countries have only really had one decent attempt to go to war - the great 1859 Pig War. Unfortunately nobody could really work up enough enthusiasm, mostly, one suspects, from embarrassment.
King Edward VIII, also while the Prince of Wales, gave the Americans reason for some concern and contingency planning with his pro-Fascist activities.