How North Korea’s missiles improved so quickly

KIM JONG UN, North Korea’s leader, has made no secret of his goal of developing a nuclear-tipped missile with enough range to destroy cities in America. What has confounded many experts is the speed at which his engineers are getting there. Only last year the consensus was that they would not succeed before 2020; now the target seems achievable within a year.

If North Korea’s sixth nuclear test, on September 3rd, was of a two-stage nuclear device small enough to fit on a Hwasong-14 intercontinental ballistic missile (ICBM), as it claims, that marks a big advance over the previous nuclear test a year ago. The regime claimed that test, too, was of a two-stage device (also known as a thermonuclear or hydrogen bomb), in which an initial nuclear explosion is used to amplify a second one. But estimates of its yield (explosive power) varied between 10 and 30 kilotons, which even at the higher end is too little for a hydrogen bomb. David Albright, the president of the Institute for Science and...

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