Does the very presence of US forces in South Korea encourage South Korean complacency and North Koream truculence, a version of the moral hazard problem?
Some pertinent thoughts from Johnathan Pearce at Samizdata on this subject, plus some excellent comments from Subotai Bahadur (my emphasis):
There is the additional possibility, raised by documented instances of nuclear cooperation between Iran and the DPRK; that the test we just watched was in fact an Iranian test using a Korean locale to avoid a response from the West. After all, they know that the world will do nothing for a Korean test; but an Iranian test would provoke a response from Israel, and once again as an extremely distant outlier, possibly by the US.
The determinative data point will be the analysis of the data collected on the detonation in the West. The DPRK makes its devices using plutonium from the Yongbyon reactor by the implosion method. The success of the test is NOT good news as it indicates that they have some mastery of the harder but more efficient method.
The Iranians use the U-235 gun type device. Their thousands of centrifuges are being used to cascade-separate and concentrate the U-235 to the 70%+ enrichment needed to make a weapon. For the record, depending on reactor design, fuel grade is 3-5%. The fissile material furnished by the Russians, it is reported by some, was already 7-9% enriched which puts paid to the idea of it being for peaceful use.
If we find that the device just detonated was plutonium based, we can make a working assumption that it was an indigenous effort. If it was U-235, we can make a similar working assumption that it was a joint Iranian-DPRK effort; with all the implications for the Middle East and the world. I also note that there is a significant incentive for the governments of the West to falsify or suppress the data if it is U-235.
Both Iran and the DPRK are operating largely under doctrines that are not rational or subject to modification by discussion in our terms; the "12th Imam" and "Juche" respectively. Their doctrines view the use of third parties to inflict deadly damage on the West as a feature not a bug.
Could a ‘bolt from the blue’ catch North Korea’s attention?
Probably. But that requires South Korea to sign up to it, and the likely vast damage ensuing to South Korea.
Which takes us back to the beginning. Does the US military presence in South Korea have the huge benefit of letting South Koreans have decent lives, but at the price of creating a zany stalemate in which North Korea does what the hell it likes?
Conventional wisdom has it that China can bring down the North Korean regime whenever it wants. But that creates risks and costs for China.
And who will cover those risks and costs? Who pays for what?
And in which currency?
Maybe this is the real Negotiation?