It finally happened: U.S. President Donald Trumppicked a fight with the nicest people on earth.

Mr. Trump’s Twitter tirade against Canadian Prime Minister Justin Trudeau following this weekend’s contentious Group of Seven industrial nations summit helped push the Canadian dollar down nearly half a percent in Asian trading hours on Monday, coming as fraught negotiations over the future of the North American Free Trade Agreement (NAFTA) appear to be at an impasse.

Mr....