Prime Minister Mark Carney says he has considered the possibility of American military action against Canada, and that the Canadian Armed Forces had examined potential responses.

Such a scenario remains extraordinarily unlikely, but that’s precisely why it belongs in contingency planning. Serious governments do not dismiss low-probability, high-consequence risks simply because they appear remote.

The first responsibility of the prime minister of Canada, as head of government, is to protect Canadians. That responsibility requires preparing not only for the most likely threats, but also for the unlikely scenarios that could have profound consequences for the country.

In intelligence and security circles, these possibilities are sometimes described using a “swan” metaphor. Preparing for likely events, or “white swans,” is routine.

Equally important, however, is considering “black swan” events: highly improbable developments that fall outside normal expectations but could have disruptive and far-reaching effects.

Why plan for black swan events?

First, contingency planning often yields benefits beyond the specific scenario being examined. By challenging assumptions and exploring unconventional........

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