America’s 1930 Plan to Invade Canada:

It was called War Plan Crimson.

Matthew Yglesias writes:

Following the 1927 Geneva Naval Conference, the US Army — evidently bored with the peace and prosperity of the 1920s — decided to draw up plans for a hypothetical war between the United States and the British Empire. The document that resulted from this exercise...was approved by the War Department in 1930 and not declassified until the 1970s. 

It assumed that Britain would start the war with a slightly larger navy and much larger army than the United States, so the key to American strategy was to start an essentially defensive effort until greater US industrial might could be brought to bear to construct a navy capable of blockading Britain. But the best way to defend the United States from a British invasion was to launch a preemptive invasion of Canada (code named Crimson), then still part of the British Empire. 

The first target was to be a quick amphibious assault on Halifax, Nova Scotia, which would deny Britain a convenient Canadian port and make it difficult for it to support Canada's military. Then two parallel invasion forces would head north from North Dakota and Vermont aimed at captured Winnipeg (a key rail junction) and Québec City (thus preventing the use of the St. Lawrence River as an alternate port) respectively.

Actual British thinking about war with the United States at the time regarded Canada as indefensible and called for the use of Bermuda and Caribbean bases as jumping-off points for attacks on American commerce.

Paul Og notes that War Plan Crimson was a subset of War Plan Red which also included othe subset plans, notably War Plan Scarlet against Australian, New Zealand and Commonwealth interests in the Pacific.

-RW