Let me go on record: I do not believe that North Korea tested a nuclear weapon or device on Sunday. I wish I had posted yesterday, but I'm buried writing a promotion evaluation of one of my colleagues and finishing a paper that's due this weekend. But ask my students: I said this before the White House has come out questioning what actually happened in North Korea.
There is very little evidence that the explosion detected in North Korea was the result of a nuclear reaction. The yield was under 1 kiloton, pathetically weak for a nuclear explosion. The bombs dropped on Hiroshima and Nagasaki had yields of 12-15kt and 20-22 kt respectively. The first devices tested by India and Pakistan in the 1990s were 12 kt and 9 kt. So, if what detonated below North Korea was, in fact, nuclear, then it either didn't work or is so primitive and rudimentary as to be almost laughable.
But why should we believe North Korea in the first place? Since the announcement that the Hermit Kingdom there has been no confirmation of the existence of nuclear weapons. US intelligence agencies agree that North Korea should have enough fissile material to make several weapons, but having the material is a long way from being able to use them. North Korea has little to gain from a policy of "strategic ambiguity" a la Israel. North Korea wants to leverage its "arsenal" against the United States to extract economic concessions and no-attack guarantees, and it can't do that with a "bomb in the basement." However, if in fact the regime is too economically crippled to build a bomb (and given that it can't even provide electricity to its capital city for 24 hours, that's not hard to believe), then and only then would it make sense to intimate that a bomb exists. Given the lessons of both Gulf Wars (don't piss of the US unless you have a nuke), North Korea likely believes that only by making the US believe that Seoul is at risk of inceneration can the US be deterred.
Of course, I may be wrong. I don't think I am, but even if I am, I don't believe that North Korea is anywhere near weaponizing a nuclear device and is, as the most recent ballistic missile tests demonstrated, even farther away from being able to deliver such a weapon. However (and this is a big caveat), the threat posed by a "nuclear" North Korea is not so much about its capability to threaten the US, but more about the political pressure it puts on the regional security dynamic. So far, China seems to be on board with the West, Japan is reassuring everyone that it will not proliferate in response, and South Korea seems to be rethinking its idiotic "sunshine" policy. So far, so good. The US needs to work to create a regional coalition and, hopefully through the UN, work to impose serious punitive consequences on North Korea. Even if there is no weapon there, such moves are necessary to reaffirm the US dominance that is necessary for tamping down regional security dilemmas.
UPDATE: Apparently, we should know in a few days with some degree of confidence whether the explosion was in fact nuclear. Read this article to see why and how.