"War Plan Red, also known as the Atlantic Strategic War Plan, was a plan for the United States to make war with Great Britain (the "Red" forces). It was developed by the United States Army during the mid 1920s, approved in May 1930 by the Secretary of War and the Secretary of Navy, updated in 1934-35, and officially withdrawn in 1939, following the outbreak of the Second World War, when it and others like it were replaced by the five "Rainbow" plans created to deal with the Axis threat. However, it was not declassified until 1974.
The war was intended to be a continental war, waged primarily on North American territory between the United States and the British Empire. The assumption was that Canada would represent the ideal geographic forum through which the United States could wage war against the British."
Canadians had a counter-plan:
"Defence Scheme No. 1 was created on April 12, 1921 and details a surprise counterattack on the northern U.S. as soon as possible after evidence was received of an American invasion of Canada.
According to the plan, Canadian troops stationed in Pacific Command in Western Canada would immediately be sent to seize Seattle, Washington; Canadian Forces stationed in Prairie Command in Western Canada would be sent to attack Great Falls, Montana and then move to Minneapolis, Minnesota; and Canadian Forces stationed in Quebec Command would be sent to seize Albany, New York in a surprise counterattack while Canadian Forces in Maritime Command[clarification needed]would counterattack into Maine. Meanwhile, according to the plan, the Canadian Forces Great Lakes Command in Ontario was assumed to be fighting on the defensive against the main attack from the Armed Forces of the USA; if Canadian forces were successful in defending in the Great Lakes area, they were encouraged to launch counterattacks in the area of the Niagara River and the St. Clair River.
When resistance stiffened, the Canadians would retreat to their own borders, destroying bridges and railways to hinder American pursuit. The purpose of the invasion would be to allow time for Canada to prepare its war effort and to receive aid from Britain, or to limit the American invasion before the US government opted to discontinue the incursions. Defence Scheme No. 1 has as a counterpart in the American War Plan Red, a plan to invade Canada drafted in 1930."
Read more on the Rainbow War Plans