The general view, however, is that miniaturized weapons are unreliable without testing. The question is whether the North Koreans attempted immediately to build a Mark 7-sized device, perhaps because they placed too much confidence in data acquired from the Soviet Union or Pakistan, or because they were just plain drunk on juche.
There is some evidence for this view, although it is admittedly circumstantial. In 2003, David Sanger reported that the United States noticed the North Koreans doing an unusual amount of testing with high explosives. The typical path toward shrinking nuclear weapons involves reducing the amount of explosives needed to compress a sphere of either plutonium or highly enriched uranium. The intelligence community, according to Sanger, concluded that North Korea was attempting to do better than a simple fission device, trying "to make nuclear warheads small enough to fit atop the country's growing arsenal of missiles."
Then, in 2005, a certain North Korean official defected. "Kim Il-do" claimed to have worked for the Second Economic Committee of the National Defense Commission, which oversees North Korea's nuclear and missile programs. Kim had some interesting things to say about North Korea's nuclear weapons program. Here is a translation from the Korean article by my colleague, Hanah Rhee:
Mr. Kim, during an interview with [South Korea's] National Intelligence Service, said that North Korea had manufactured a one-ton nuclear warhead with four kg of plutonium. He also said "North Korean scientists reported to Kim Jong-il that the weapon is functional, however, they have doubts about the level of performance that they manufactured." Mr. Kim said, "North Korea is not confident whether their large-sized nuclear weapons can actually work in practice which is why they are developing a smaller 500 kg version."
At the time, I was inclined to doubt the guy. That's not how the United States, or anyone else, did it. And for good reason -- if North Korea were to test such a weapon, the result would probably be a humiliating failure.
Then North Korea tested a humiliating failure.
North Korea told China to expect a four-kiloton yield. That's broadly consistent with what one might expect from a device with only four kilograms of plutonium. The actual result was a few hundred tons' worth of yield. Not something you'd want dropped on your neighborhood, to be sure, but also not something you'd be eager to explain to the Dear Leader. Suddenly, at least to me, Kim Il-do didn't seem so crazy.
If you believe this second theory of North Korea's strategic development, then North Korea isn't incompetently following the U.S. or Soviet nuclear weapons development path. With fewer tests, North Korea is trying to move more quickly to larger, deliverable warheads based on the experience of others. Although I suspect I'm in the minority, I still believe that the North Koreans' ultimate goal is a stockpile of missile-delivered thermonuclear weapons. Broadly speaking, I suspect this test is a step toward that end. There are many technical details that we don't know at the moment -- and may never know. We don't know yet whether the device used plutonium, highly enriched uranium, or both, and we might never know. We also don't know how North Korea made a bigger bang this time -- whether the progressively larger explosions represent improvements in the existing approach or changes in the entire design philosophy.