Canada and the United States might seem like the best of friends now, but that hasn't always been the case. Tensions once ran so high between the U.S. and its northern cousin that each country created their own invasion plan — just in case.

Canada started it.

In 1921, a Canadian lieutenant by the name of Buster Brown drafted "Defence Scheme No. 1." Despite "defense" in the title, it was "a full-on invasion plan," according to Kevin Lippert, the author of "War Plan Red." Lippert joined MPR News' Tom Weber to discuss the actual plans behind the theoretical invasions.

Brown and his fellow officers, Lippert writes, "donned disguises, loaded into their Model T, and began an espionage mission along the Canada-New England border." Brown photographed bridges, highways and other sites of interest in the northeastern U.S. He also interviewed local residents to test where their allegiances lay.

In the end, he proposed a five-pronged attack. In the west, Canadian troops would take Seattle and Portland. In the east, the Quebecois would occupy Albany. Maine would be reclaimed, as would the Great Lakes. In the Midwest, Brown's plan called for "Prairie Command" to swing through Fargo and then head south to invade Minneapolis and St. Paul.

"It was a series of lightning raids into the United States, then they would withdraw, blowing up roads and bridges behind him to buy enough time for the British to sail to his rescue," Lippert said of Brown's plan.

Defence Scheme No. 1: A Canadian lieutenant fashioned a five-pronged plan of attack against the U.S. in 1921. Courtesy of Princeton Architectural Press

Obviously, it never came to fruition. The tensions driving the potential invasion were actually between the U.S. and Britain. Canada was simply a proxy. After World War I, Britain owed the U.S. a tremendous amount of money — approximately $22 billion — and there were intense disagreements over the payment terms.

The disagreements were heated enough for the U.S. to draft a plan of its own in 1930, which it dubbed "War Plan Red."

"Americans at that time, everybody seemed to think it was just a matter of time before Canada would be absorbed into the U.S.," Lippert said.

The American plan was an uncanny mirror image of the Canadian plan. U.S. troops would invade from St. Paul into Winnipeg, from Albany into Quebec, from Boston into Halifax. The goal was to push Britain out of Canada and to create a "United States of North America."

The top-secret plans remained locked-up until the 1970s, except for one brief leak of the U.S. plan to the New York Times — but Canadian intelligence apparently didn't notice, Lippert said.

After World War II, the plan seemed preposterous. The two countries united behind one nuclear shield with NORAD, which today is commanded by one American general and one Canadian general. With united militaries, the country's economies are similarly linked: each is each other's largest trading partner.

These co-dependencies make an invasion unthinkable — except in comedy. The 1995 John Candy movie "Canadian Bacon" and the 1999 "South Park" movie both riffed on a potential war between the countries.

But Lippert doesn't see life imitating art anytime soon.

"At this point invading Canada would be like setting a fire in our own living room," he said.