After trade talks with Canada collapsed last week, the US announced new tariffs of 50% on hundreds of products, Canada promised to retaliate “dollar for dollar,” and the White House said it would meet any such response with further escalation. In short, one of the world’s closest trading relationships is about to break down at enormous cost to both countries — and for no intelligible reason.

Just why the talks failed is disputed. Canadian Prime Minister Mark Carney suggested the US made unacceptable last-minute demands, including measures that would “compromise Canada’s sovereignty or undermine our key industries.” (Reportedly these included limits on the country’s French-language protections, restrictions on its other trade deals and access to its critical minerals.) US Trade........

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