At about 3:30 zulu time, radio hobbyists monitoring a U.S. military frequency (4724z) noted a 249 character emergency action message emanating from Yokota Air Base. At about the same time, seismic stations across the world picked up an earthquake of magnitude 5.1. The point of origin seemed to be a swath of land on the Korean peninsula where North Korea has previously tested nuclear weapons. A few hours later, the North Korean official news agency confirmed what the electromagnetic spectrum (and numerous U.S. spy satellites and sensors) already reported: It had set off a controlled underground explosion of nuclear material.
A test has been rumored for months; the U.S. and South Korean intelligence apparatuses seem to have gotten good at projecting these things. Aside from saber-rattling, though, what does the DPRK's third nuclear test mean?
(1) China has moved troops and artillery to its shared border with North Korea. It has warned the country in public not to conduct its test. Since China rarely criticizes its client state in public, the Chinese pressure might mean that it will be on board with a more forceful response to Pyongyang. What type of sanctions will China agree to? Given how poor North Korea is, any financial sanction regime has the potential to hurt innocent people. There is no real "middle class" to speak of, unlike in Iran.
(2) Japan, spooked because China had moved its troops, moved its troops. Japan's government is very nationalist. And South Korea's new prime minister, who had talked of rapprochement, will now have to take a harder line.
(3) What is the composition of the bomb? Nuclear scientists will be able to determine fairly quickly whether the critical nuclear material was plutonium and uranium. They will also get a good sense of the yield by comparing the seismic information with the amount of radiation (all across the electromagnetic spectrum).
(4) What does President Obama say about North Korea during the State of the Union? (Remember, President Bush used his second SOTU to include North Korea among the "axis of evil.") Does the administration privately assume the test is for North Korean domestic consumption?
(5) Does North Korea twin its nuclear test with a missile test? If so, is the missile different than previous missiles known to have been tested?