Two of the world's most intertwined economies now face each other across a widening tariff divide. Canada, responding to duties imposed by the Trump administration, has announced retaliatory measures on American goods — a move that transforms an uneasy trade tension into a formal cycle of economic retaliation. What unfolds between these deeply integrated neighbors will not stay at the border; it will travel inward, touching supply chains, factory floors, and kitchen tables across North America. History suggests that tit-for-tat trade disputes rarely end where they begin.