[Andrei Lankov is a professor at Kookmin University in Seoul and the author of several books on North Korea.]

SEOUL — Last week, Siegfried Hecker, a former director of the Los Alamos National Laboratory was invited to visit the North Korean nuclear research center in Yongbyon. He was shown a uranium enrichment plant whose sophistication and likely output is well in excess of what most experts suspected about the North Korean uranium program. Then on Tuesday, North Korean artillery shelled a South Korean island, inflicting heavy damage.

The world is likely to say that the North Koreans are again acting “irrationally.” But this is not the case — they are a very rational regime, actually the world’s most Machiavellian.

North Korean leaders are sending a message. For the last two years, both Washington and Seoul have tried to ignore them, so now they use both artillery and centrifuges to say: “We are here, we are dangerous, and we cannot be ignored. We can make a lot of trouble, but also we behave reasonably if rewarded generously enough.”...