The United States and Canada, by and large, have been peaceful neighbors — especially since Canada became a de facto independent nation under the British North America Act in 1867. But while the two nations are friendly and, typically, allies, things can change. And in 1927, the United States planned for just such a scenario.
At the time, Canada was still mostly under British control, and even though the United States and the United Kingdom were friendly — they fought on the same side in the Great War — things could always change. The U.S. was concerned that the UK’s imperial desires might extend back to the U.S., and the U.S. was not going to be caught unprepared. The U.S. Army developed “War Plan Red,” a comprehensive strategy to foil any British expansion into its long-former colony.
War Plan Red assumed that, in the case of war, Britain had two significant advantages. First, the British navy was a formidable force, able to control the seaways and therefore the U.S. export economy. Second, the UK controlled Canada, and could have used it as a staging ground for an invasion of the United States. The American plan was to strike Canada first.
Specifically, U.S. forces would invade Nova Scotia, hoping to take Halifax, which (American strategists assumed) would be the focal point for the British Navy in North America. If this failed, the U.S. would try to take New Brunswick, isolating Nova Scotia from the mainland. After securing that region, American forces would target Quebec City, further separating east from west; Ontario, taking control of much of Canada’s manufacturing (at the time); Winnipeg, a railway transit hub; and Vancouver, as part of a belt-and-suspenders approach toward controlling the ports. War Plan Red only laid plans for military action in the Western Hemisphere — America never intended to attack the British Isles. Rather, the plan was to hold Canada hostage, so to speak, in hopes that Britain would agree to a peace treaty to free its largest New World territory.
In 1974, the United States declassified War Plan Red. which created a minor ripple in U.S./Canadian relations — but it quickly passed.