One of the world’s closest trading relationships is about to break down at enormous cost to both countries — and for no intelligible reason. …Canadian Prime Minister Carney suggested the US made unacceptable last-minute demands, including measures that would “compromise Canada’s sovereignty or undermine our key industries.” US Trade Representative Greer said Canada inexplicably turned its back on improvements that would’ve made a good deal for both sides even better. That claim was hardly credible. This White House has shown no interest in mutual advantage. …American voters, on the other hand, may be unwilling to bear any burden merely to show Canada who’s boss. …US businesses are puzzled too. Many American companies have supply-chains tightly linking domestic and Canadian operations. …But it isn’t too late to step back. Washington should revert to the terms in play before the talks collapsed. …If the worst happens, who are Americans heading to the polls in November going to blame?