First of all, let me just say that I don't think it was a nuke test...of course I'm not 100% sure, but I just don't think it is.
However if it is a nuclear test (and we all know they have nukes), who can blame them?
George W. did in fact begin a dangerous new arms race. In 2001-2002 (started BEFORE 9/11), Dubya's administration pushed Congress into withdrawing from at least half a dozen arms-related international treaties. I'm too lazy to look for links right now, but I'll tell you one of those treaties was the ABM treaty with Russia. The entire world, including Republicans in the US begged them not to do this, but they went ahead and did it anyway.
The reason for Bush withdrawing from all these treaties was primarily because 90% of his administration and family are members and CEOs of weapons manufacturing companies (such as Carlyle for one).
Well...finally Bush said the fuck with everyone and withdraw from the treaties. Then other countries, but most importantly Russia said "ok, if you're gonna go make new weapons then so will we". So the Bush adminsitration went ahead and spent 80 billion of taxpayer dollars on developing a 20 year old missile defense system (a toy) that is absolutely useless today. In the meantime, Russia has developed a new ultrasonic type of missile (and they're almost done with yet another one) that is completely involnerable to any defense technologies we have. The department of defense and the military early this summer were forced to admit that yeah, our 80 billion dollar toy can't do anything to it (or for that matter to any other modern missiles used today).
What's more is the chain of countries involved in the arms race. If we develop new weapons, then so does Russia (and hey, they just did!). If Russia develops new weapons then so does China. And if China does, then so does North Korea.
In the meantime, North Korea has gotten even more incentive to start arming themselves. See, in January 2002 Dubya declared Iraq, Iran, and North Korea as "evildoers" part of an Axis of Evil. Nobody thought that he would actually go in and attack one of them without just and provable cause but hey - he did!
So now imagine you're the crazy Kil Jon Il of North Korea. You see the US creating new weapons. You see Russia creating new weapons. Then you hear yourself declared as an evildoer by one of the countries making weapons. And finally you see one of the 3 countries Bush declared as evildoers checked off on his list. Wouldn't you start developing your own nuclear arsenal?