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In the 1920's several "rainbow" war plans were drawn up each with plans to go to war with a different country. One of them was War Plan Red which was a detailed list of milestones to be met in a war with Great Britain.

The main zones of operation discussed in the plan are:
Nova Scotia and New Brunswick:
Occupying Halifax, following a poison gas first strike, would deny the British a major naval base and cut links between Britain and Canada.
The plan considers several land and sea options for the attack and concludes that a landing at St. Margarets Bay, a then undeveloped bay near Halifax, would be superior to a direct assault via the longer overland route.
Failing to take Halifax, the U.S. could occupy New Brunswick by land to cut Nova Scotia off from the rest of Canada at the key railway junction at Moncton.
Quebec and the valley of the Saint Lawrence River:
Occupying Montreal and Quebec City would cut the remainder of Canada off from the Eastern seaboard, preventing the movement of soldiers and resources in both directions.
The routes from northern New York to Montreal and from Vermont to Quebec are both found satisfactory for an offensive, with Quebec being the more critical target.
Ontario and the Great Lakes area:
Occupying this region gains control of Toronto and most of Canada's industry, while also preventing Britain and Canada from using it for air or land attacks against the U.S. industrial heartland in the Midwest.
The plan proposes simultaneous offensives from Buffalo across the Niagara River, from Detroit into Ontario, and from Sault Ste. Marie into Sudbury. Controlling the Great Lakes for U.S. transport is considered logistically necessary for a continued invasion.
Winnipeg
Winnipeg is a central nexus of the Canadian rail system for connecting the country.
The plan sees no major obstacles to an offensive from Grand Forks, North Dakota, to Winnipeg.
Vancouver and Victoria:
Although Vancouver's distance from Europe reduces its importance, occupying it would deny Britain a naval base and cut Canada off from the Pacific Ocean.
Vancouver could be easily attacked overland from Bellingham, Washington, and Vancouver Island could be attacked by sea from Port Angeles, Washington.
The British Columbia port Prince Rupert has a rail connection to the rest of Canada, but a naval blockade is viewed as easy if Vancouver were taken.

This plan was never initiated because of WWII. But what if the second world war didn't happen or was at least postponed?
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