Several factors – all driven by Pyongyang – are at work and have created an increasingly tense military situation on the peninsula. Additional long-range missile tests are expected, and naval clashes along the Northern Limit Line (NLL) are becoming more likely.
Political
Some background: In January 2009, North Korea “voided” all political-military agreements with [...]

That someone in the administration recognizes the absurdity of engaging in Ground Hog Day deals with North Korea (and is publicly commenting about it) is somewhat encouraging. Robert Gates, U.S. Secretary of Defense, recently had this to say about North Korea:
“We will not stand idly by as North Korea builds the capability to [...]

The title assumes North Korea did test a nuclear device, which I think likely, and not an equivalent amount of TNT, initially thought a possibility in 2006 until radioactive isotopes of krypton and xenon were detected.
There are several reasons for a North Korea to conduct a nuclear test and it’s difficult if not impossible [...]

Update, North Korea’s announcement: Full text of the Korean Central News Agency (KCNA) announcement of the nuclear test, presumably a TV broadcast (via the Independent):
“The Democratic People’s Republic of Korea successfully conducted one more underground nuclear test on May 25 as part of the measures to bolster up its nuclear deterrent for self-defence in [...]

The Taepo Dong 2 (TD-2) (대포동 2호) missile launch(es) North Korea likely is preparing for, and recent voiding of all political-military deals with South Korea, are part of Pyongyang’s long-term strategy of regime survival. While these actions also have less important near-terms goals – expressing displeasure with ROK President Lee Myung-bak’s relatively hard-line approach [...]

In mid-2008, North Korea turned over 18,000 pages of documents to U.S. officials to help prove it was telling the truth about its record of plutonium processing. Ironically, those documents had trace amounts of highly enriched uranium (HEU) that was likely not possible transfer from Pakistani equipment obtained via A.Q. Khan. The North [...]

Update: Probably it would have been better to get these answers *before* announcing plans to delist North Korea and remove other trade barriers, as that is what the agreements actually called for:
North Korea did not answer U.S. suspicions of enriching uranium and proliferating technology when it released an inventory of its nuclear plans this [...]

This is almost too good to be true - according to U.S. intelligence sources, the physical records of plutonium processing North Korea gave the U.S. for determining how much had been processed had trace amounts of highly enriched uranium (HEU) on them, and likely not possible transfer from Pakistani equipment supplied by prolific proliferator A.Q. [...]

According to author Shyam Bhatia, who is said to have known former Pakistani Prime Minister Benazir Bhutto well:
FORMER Pakistani prime minister Benazir Bhutto, on a state visit to North Korea in 1993, smuggled in critical data on uranium enrichment — a route to making a nuclear weapon — to seal a missile deal with [...]

A classic example of North Korean negotiating tactics; this one never grows old:
North Korea questioned Saturday U.S. willingness to settle an ongoing nuclear dispute after criticisms by Washington included in a report on global democracy.
North Korea’s official news agency said the country’s Foreign Ministry blasted a recent State Department report that called North [...]