At the stroke of midnight, Canada crossed a threshold that few neighbors ever wish to reach, imposing retaliatory tariffs on American goods in answer to earlier U.S. trade measures. The move is less a beginning than a deepening — another turn in the long, entangled economic relationship between two countries whose fortunes are so intertwined that a tax at one border becomes a price increase in the other's grocery aisle. History reminds us that trade disputes between close partners carry a particular weight: the very integration that made both nations prosperous now amplifies every friction. Wh