As Canada's retaliatory tariffs on American goods took effect, Republicans gathered in Dallas for their midterm convention — two events that together mark a moment when the costs of economic nationalism become impossible to ignore. The deep interdependence of the North American trading relationship means that each new tariff is not merely a policy instrument but a disruption felt in factories, on farms, and at checkout counters. History reminds us that trade wars rarely have clean victors, and the political reckoning tends to arrive before the economic one.