As one of the roughly 800,000 Canadians living full-time in the United States, my jaw dropped when I first heard about Donald Trump “joking” that Canada ought to be the 51st state. I was horrified. As a professor of international relations, however, I’m familiar with the long history of American threats against Canada’s sovereignty, dating all the way back to the American Revolution.

The U.S. has invaded us twice—first in 1775, and again during the War of 1812. Back then, Canada was just a collection of colonies. It survived largely thanks to its Indigenous allies in North America and backing from the British Empire. Aside from a few close calls in the 1800s, Americans have decided against conquering us for the last 200 years, despite the clear military imbalance between us. (The U.S. spends 35 times as much on the military as we do.) Over the last year, as part of a larger project on the formation of Canada, I’ve researched why and how we’ve managed to stay out of America’s clutches for so long. As it turns out, history can provide us with a lot of lessons for how to best react to Trump’s current threat.

You might think that past annexation plots have failed because Americans just plain like Canadians, and it’s hard to go to war against your friends. There is some truth to that. A YouGov poll from January showed that 76 per cent of Americans have “very or somewhat favourable” views of Canada. (The same poll revealed that Canadians have a less rosy view of Americans: 56 per cent reported having a negative opinion of the States, which Trump is obviously not helping.) Even if some U.S. politicians—say, an orange-coloured president—wanted to invade, they’d still face the challenge of persuading a reluctant public. Wars are bloody and costly affairs. American voters know it, and elected officials, for all their faults, tend to pay attention to them.

On its own, though, public opinion is an unreliable political force. Sometimes that’s........

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