A good place to start might be the Foreign Ministry statement of April 29:
In case the UNSC does not make an immediate apology [for the presidential statement condemning the launch of the Unha-2], such actions will be taken as:
Firstly, the DPRK will be compelled to take additional self-defensive measures in order to defend its supreme interests.
The measures will include nuclear tests and test-firings of intercontinental ballistic missiles.
Secondly, the DPRK will make a decision to build a light water reactor power plant and start the technological development for ensuring self-production of nuclear fuel as its first process without delay.
We are now at one nuclear test and counting.
(Why) Did It Come as a Surprise?
In light of the foregoing statement, today’s test cannot have come as a surprise to anyone. But people who follow this subject intently were taken aback by how soon it happened. One would assume that the preparations were in motion even before April 29, yet we saw nothing in the papers about it. That’s awfully interesting, since the last time a nuclear test was announced to the world as a fait accompli — I’m relying on memory here, so please correct me if I’m wrong — was the first of India’s two rounds of testing in 1998, widely considered in the United States to have been an intelligence failure.
There are two possibilities. Either A) the Obama Administration saw some advantage to keeping mum, and turns out to be awfully good at keeping mum, or B) someone missed something they should not have missed. If it’s the latter, the results may be no more than mildly embarrassing, but it’s still a little disconcerting.
Update: Chosun Ilbo reports that the U.S. and South Korea were keeping a weather eye on the test site. But it’s not clear that they had good indications on timing.
Further update: Thanks to the contributions of readers here and here, it’s clear that Option A, above, is the correct answer. There were a few leaks, but nothing that the community of wonks picked up on the time. Perhaps Option B applies to us. We’ll have to do better, next time.
I had not seen it widely discussed, but would venture that the tacit consensus, expressed earlier by Sig Hecker, was that North Korea was unlikely to test again before completing a reprocessing campaign. Perhaps not, after all.
Now might be a good time to revisit what North Korea is doing on ICBMs and the front end of the nuclear fuel cycle.