Two neighboring nations bound by the longest undefended border in the world now face each other across a widening economic divide. Canada's retaliatory tariffs, covering $20 billion in American goods at rates between 15% and 50%, took effect in the early hours of Tuesday morning — a measured but unmistakable answer to Washington's own escalating demands. What began as a dispute over trade balances has grown into something more revealing: a contest over sovereignty, language, and the terms on which nations may govern themselves. History suggests that trade wars rarely end cleanly, and this one