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.”

Since 1994, U.S. policy toward Pyongyang has been based most of the time on the assumption that North Korea can be persuaded and bribed into surrendering its nuclear program.