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In page Canada–United States relations:

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During the 1920s, the war and naval departments of both nations designed war game scenarios with the other as an enemy as part of routine training exercises (neither military was ever told to get ready for a real war[citation needed]). In 1921, Canada developed Defence Scheme No. 1 for an attack on American cities and for forestalling an invasion by the United States until British reinforcements could arrive. Through the later 1920s and 1930s, the United States Army War College developed a plan for a war with the British Empire waged largely on North American territory: War Plan Red.[4]