Choe Sang-hun, reporting for the New York Times, speculates that North Korea's nuclear test surprised the United States and South Korea:
The test appeared to have caught South Korea and the United States off guard, and the news hit just as South Korea’s government and people were mourning the suicide of former President Roh Moo-hyun.
If officials were caught off guard, it wasn't because they weren't expecting a nuclear test. News organizations had been reporting on preparations for a possible test for weeks, citing South Korean officials. Then, of course, there is the fact that North Korea had also been warning it would do exactly this since April.
North Korea may, however, have pulled off its test earlier than expected. Experts mistakenly thought it would take weeks to make all the necessary preparations, as was the case when North Korea conducted a less-successful test in 2006.
"North Korea seems to want a speedy game," one senior South Korean official told Yonghap. "It seems to be seeking to create a condition favorable to itself as early as possible, rather than dragging its feet."
“The suddenness of the nuclear test shows North Korea following military, not diplomatic logic,” Hideshi Takesada, a Korea expert at Japan’s National Institute for Defense Studies, told Bloomberg.
Kenneth Quinones, a former North Korea director at the U.S. State Department, thinks the launch means that the generals are running this show.
"They’ve convinced Kim to bulk up their military capabilities in advance of any diplomacy,” Quinones told Bloomberg. “But they’re painting themselves into a corner."
I'm not sure what Quinones means by that, but from past experience, the North Koreans have to be thinking that their position going into any talks is going to be stronger now. Their first nuclear test in 2006 was most likely a dud, but it brought the Bush administration to the table. Imagine the goodies they'll get now that, as it appears, their device actually works?
Interestingly, South Korean presidential spokesman Lee Dong-kwan said that North Korea may have even notified the United States ahead of time, and a "diplomatic source in Beijing" told reporters that China was given a head's up as well.
Also noteworthy: North Korea fired off three surface-to-air missiles after its test, two of which were reportedly a warning to U.S. spy planes to back off, according to Yonhap.