In the 1920s, the U.S. military devised a contingency plan for attacking Canada. After a first strike with poison gas, we’d occupy Halifax, invade Montreal and Quebec from New England, strike at the Great Lakes from Detroit and Buffalo, and impose a naval blockade on British Columbia.
At the same time, Canada’s Col. James Sutherland Brown developed a counter-invasion strategy where flying air columns would occupy Seattle, Portland, Oregon, Minneapolis, and St. Paul. Gen. George Pearkes called it a “fantastic desperate plan [that] just might have worked,” but it was withdrawn in 1931.