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.