OH CANADA….The idea of an American invasion of Canada is not entirely new. The U.S. first gave it a shot in 1775, and tried again during the War of 1812, but the campaigns were never successful. A couple of centuries later, we can all look back at the “incidents” as quaint misadventures.

But as the Washington Post’s Peter Carlson noted today, as recently as the 1930s, the U.S. government had a frequently updated plan for a Canadian invasion. It was a 94-page document called “Joint Army and Navy Basic War Plan — Red,” with the word SECRET stamped on the cover. The plan was to seize Canada by capturing Halifax, then taking control of Canadian power plants near Niagara Falls, and finally launching a three-front invasion through Vermont, North Dakota, and the Midwest. The whole thing sounds more detailed than Bush’s plan to invade Iraq.

It sounds like a joke but it’s not. War Plan Red is real. It was drawn up and approved by the War Department in 1930, then updated in 1934 and 1935. It was declassified in 1974 and the word “SECRET” crossed out with a heavy pencil. Now it sits in a little gray box in the National Archives in College Park, available to anybody, even Canadian spies. They can photocopy it for 15 cents a page.

Apparently, war planners expected a long, drawn-out battle that might ultimately lead to Canada demanding Alaska.

But before anyone condemns 1930s America for its military ambitions, it’s worth noting that Canada had an invasion plan of its own.

In fact, Canadian military strategists developed a plan to invade the United States in 1921 — nine years before their American counterparts created War Plan Red.

The Canadian plan was developed by the country’s director of military operations and intelligence, a World War I hero named James Sutherland “Buster” Brown. Apparently Buster believed that the best defense was a good offense: His “Defence Scheme No. 1” called for Canadian soldiers to invade the United States, charging toward Albany, Minneapolis, Seattle and Great Falls, Mont., at the first signs of a possible U.S. invasion.

Fortunately, we can all get along now. “Canadian Bacon,” “South Park: Bigger, Longer & Uncut,” and Jonah Goldberg notwithstanding, no one seriously believes there will be a military conflict between the U.S. and Canada. Well, almost no one.

Floyd Rudmin is a Canadian psychology professor and author of “Bordering on Aggression: Evidence of U.S. Military Preparations Against Canada.” Apparently, he’s a little concerned about Fort Drum, the huge Army base in Upstate New York.

Is he worried that the Yanks will invade his country from Fort Drum?

“Not now ,” he said. “Now the U.S. is kind of busy in Iraq. But I wouldn’t put it past them.”

He’s not paranoid, he hastened to add, and he doesn’t think the States will simply invade Canada the way Hitler invaded Russia.

But if some kind of crisis — perhaps something involving the perennially grumpy French Canadians — destabilized Canada, then . . . well, Fort Drum is just across the river.

Jon Stewart recently joked that Canada has nothing to worry about unless the United States runs out of natural resources. He was kidding. I think.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.