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President Trump has promised the world that he will “solve” the North Korean nuclear crisis before the country’s leader, Kim Jong-un, can screw a nuclear weapon onto a missile that can reach San Francisco or Los Angeles, a grim feat that experts say he is on track to achieve during Mr. Trump’s first term. The president is right to point out that his predecessors succeeded only at kicking this problem down the road. But Mr. Trump hasn’t said how he plans to solve the problem.

History suggests that as Mr. Trump comes to understand the risks involved, he will settle for constraints on North Korean testing…