'Nuclear Showdown' and 'The Coming Collapse of China' author Gordon Chang provides insight about why and how North Korea performed another nuclear test:
GORDON CHANG: Really, we have been disrespected, and now we have seen all sorts of missile tests -- one over Japan on Tuesday. This is really serious for our friends in the region and it challenges the credibility of the United States...
This is psychological, of course, the North Koreans are making very fast progress. One of the things we have to be asking, not only with nukes, but with the missiles, is: They have to be getting help. The leat likely explanation is that they're doing this themself.
We need to know who is giving them the technical assistance, plus also the material, as well as diplomatic, economic, and money-laundering support. That is necessary to be able to do 120 kilotons.