The Pentagon does have a plan to invade Canada, although apparently it hasn't been updated since 1935 (at least as far as we know). It was declassified in 1974. According to the Washington Post, the plan roughly goes like this:
First, we send a joint Army-Navy overseas force to capture the port city of Halifax, cutting the Canadians off from their British allies. Then we seize Canadian power plants near Niagara Falls, so they freeze in the dark. Then the U.S. Army invades on three fronts -- marching from Vermont to take Montreal and Quebec, charging out of North Dakota to grab the railroad center at Winnipeg, and storming out of the Midwest to capture the strategic nickel mines of Ontario. Meanwhile, the U.S. Navy seizes the Great Lakes and blockades Canada's Atlantic and Pacific ports. At that point, it's only a matter of time before we bring these Molson-swigging, maple-mongering Zamboni drivers to their knees! Or, as the official planners wrote, stating their objective in bold capital letters: "ULTIMATELY TO GAIN COMPLETE CONTROL."
http://www.washingtonpost.com/wp-dyn/content/article/2005/12/29/AR2005122901412.html