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 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 World War — things could change quickly. The U.S. was concerned that the UK’s imperial desires, albeit unlikely, could extend back to the U.S., and the U.S. was not going to be caught unprepared. The American Army, therefore, 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.
In 1974, the United States declassified War Plan Red. which created a temporary ripple in U.S./Canadian relations — but it quickly passed.
From the Archives: The Pig War: In 1859, the U.S. and Canada (still fully controlled by the UK) almost went to war. There were shots fired — and there was one casualty.
Related: “The U.S. of EH?: How Canada Secretly Controls the United States and Why That’s OK” by Kerry Colburn and Rob Sorensen. Seven reviews, four stars on average.