DURHAM, N.C. — The worst thing you could say about previous American presidents and their sleepy approach to Canada was that they took their polite northern neighbors for granted.

But as President Trump jetted away from the wreckage of the Group of 7 summit meeting in Quebec this weekend, he plunged American-Canadian relations into a dive so steep it provoked nosebleeds on both sides of the border.

He called the Canadian prime minister, Justin Trudeau, dishonest and weak. His advisers accused the Canadians of stabbing Mr. Trump in the back as he winged his way to a meeting with his new best friend, Kim Jong-un of North Korea. And one aide said a “special place in hell” was reserved for Mr. Trudeau, who had the temerity to say to Mr. Trump what the president likes to say to everyone else: Don’t push me around.

It’s not every day you see an American president trade a two-century relationship with a reliable neighbor for what could amount to a one-night stand with a ruthless dictator in Singapore. Mr. Trump may well think bullying Canada is cost-free. After all, three-quarters of its exports go to the United States, which makes retaliation risky for Canada. But having limited options does not mean having none. Reversals like these come with a price, although how and when the United States will pay depends on many factors.