It sounds far-fetched these days, but as recently as the 1920s, the director of Military Operations and Intelligence at Canadian army headquarters drafted a plan calling for such a tactic.

James Sutherland "Buster" Brown had joined the army in 1906. He served overseas in World War I and in the 20s took up the director's post where he drafted Defence Scheme Number 1. Under the plan, Canadian mobile columns would strike southward into the U.S. to capture such "key" bases as Seattle and Minneapolis. This would stall the American army from moving northward and allow time for the British army to arrive and help Canada.

Brown was descended from Loyalists and was described as "deeply suspicious" of Americans. Although few people, including soldiers, took the scheme seriously and Brown was later dismissed from his post, Canada had long had a fear of American invasion throughout the 19th century and up to World War I. And according to one historian, the Americans had a hypothetical plan for conquering Canada in the 20s as well. It was called the U.S. Army's Strategic Plan Red.

Kind of makes you think twice about free trade.