Elsewhere in the "Axis of Evil"
North Korea tested a plutonium weapon last night. Technically, it is of, at most, moderate significance: the real hurdle is getting the plutonium, and it has been known for many years that DPRK had the material. Plutonium is moderately tricky to weaponize, so any technical significance lies in the demonstration of their ability to weaponize it. But "moderately tricky" in this context means "merely a matter of modest time and effort", not "maybe they won't be able to do it".
I've read reports that the weapon system itself was 10 tons. That is an undeliverable weapon by conventional means. Which is to say, North Korea will not be raining nuclear missiles, or flying nuclear bombers, over Tokyo anytime soon. But miniaturization is, again, simply a matter of time and effort. And unconventional delivery is always a (risky) option.
Image from armscontrolwonk's post on the likely location of the DPRK test site.
From an overall policy point of view: as has been pointed out many, many, many, many times, a military strike on Iran might set its nuclear program back three to five years at most. The same military-strike crowd despises the Clinton program on DPRK, which delayed their nuclear capabilities by the same length of time, at an extremely low cost. Achieving a fixed goal at low (strategic, human, capital) cost is what good policy is all about.
The political significance is much greater and I leave it to others to address (sorry, TPM doesn't have individual article links today due to server migration). I'm especially looking forward to the whirledview and armscontrolwonk takes. But I do want to make a short note. Five years ago, there was absolutely no strategic alignment between the activities of Iran, Iraq, and North Korea. That changed with the "Axis of Evil" speech. Competent leaders typically try to divide their enemies.
Updated: since I wrote this post this morning it has become increasingly clear that the yield is sub-kiloton. I've not had time to learn any more than what I've heard on NPR; but offhand I'd guess that a sub-kiloton yield is actually difficult to obtain intentionally. Which is to say, I don't think it likely they intended to test a sub-kiloton yield. There has been some speculation this could be a faked test with conventional weapons. As far as I know, no nation has ever detonated this much conventional explosive at once. The U.S. scheduled a test ("Divine Strake") this past summer for a similar quantity, but cancelled it.
If it really is a misfire (this is one of the moderately tricky things about plutonium), then that's good news of a sort. I've never heard of a nation actually failing a first fission test shot. (Of course, it's not my primary area of expertise, either, so maybe I missed one.)
Update 2: I came across this interesting Morimoto article on the consequences of a DPRK nuclear test on underwater systems in the Korean Peninsula (h/t globalsecurity). The article starts on page 8, the discussion of the water on p. 14. Also, AJ at Americablog passes along that his sources say there has been radiological confirmation.
Update 3: armscontrolwonk has weighed in with multiple posts; also, whirledview has multiple posts. armscontrolwonk confirms that indeed, no nation has ever failed a first test shot, and declares, "North Korean nuclear scientists are now officially the worst ever."