There is a site called 'Damn Interesting', that has posted an article called 'America's Secret Plan to Invade Canada'. Here is an excerpt from the article "At a length of 5,522 miles (8,891 kilometers), Canada and the United States share the longest non-militarized border in the world. Today we think of the two nations as the friendliest of neighbors, but at one time both nations had somewhat detailed plans for attacking one another... just in case."
I was not sure if this story was true, but I did some research, and several sources confirmed the authenticity of it. Below is a reference to another source confirming the story.
Raiding the Icebox
Behind Its Warm Front, the United States Made Cold Calculations to Subdue Canada
By Peter Carlson, Washington Post Staff Writer
Friday, December 30, 2005; Page C01