Two neighboring nations bound by decades of economic integration now find themselves caught in an accelerating cycle of trade retaliation, each measure inviting the next in a logic that serves neither side well. Beginning Tuesday, Canada will impose tariffs as high as 50 percent on American goods — a response born not of confidence but of the difficult calculus that retreat can cost more than resistance. Washington's renewed threats of further escalation suggest that the machinery of this dispute has developed a momentum of its own, and that the deeper question is no longer who struck first, b