Its not just political, its military as well. Or rather, the political situation is determined by the military situation. North Korea would make a mess out of South Korea in any war, even a victory. Only a pre-emptive nuclear strike would provide for a relatively intact South Korea. That is politically impossible. And South Korea isn't going to support any move by the US that might result in massive damage to it. Thus the current strategy of waiting until North Korea falls apart.

and due to fallout, even a nuke strike on N.Korea would leave S.Korea plenty bad. Seoul is a massive densely populated city very close to the NK border...not a long trip for fallout.

How about this: The main player in this situation is the Republic of Korea. In the event of another Korean War, they will suffer far more than we, and their economy, which is roaring along right now, could be devastated. Even if the war is quick and "easy", guess who gets: 1) To pick up all the pieces 2) To feed and educate 20 million starving, functionally illiterate north Koreans 3) Brand new borders with Russia and China This is not something the ROK takes lightly. If THEY do not wish to attack north Korea, that is an important reason for us not to. People here tend to ignore that the Koreans, and not us, have by far the greater stake here. It was like that in 1950, too, but you'd never know it from our media. I was quite interested to learn, during my tour of duty there, that the Koreans in general were not at all impressed by the TV series MASH. Why? Because MASH always potrayed the Koreans as simple minded innocent victims, and never as the major protagonists. In 1950, with the north Korean Army advancing rapidly, and refugees pouring out of Seoul across the Han Gang bridges, the RoK government decided to blow up the bridges. They decided this had to be done, because the Korean and allied forces needed every minute they could to establish a defensive perimeter and foothold at Pusan. They could not clear the civilians from the bridges. They blew them anyway, because failing to do so would have cost them the war. Hundreds died, the north Koreans were temporarily halted, the Pusan perimeter was ready only barely in time, and it held when it had to. But you don't ask people to face decisions like that lightly..

geopolitics too,russia and china with an added emphasis on china.

SGTOvious told Korean viewpoint about the question – why (US) don’t take military action against N. Korea. So, I’ll say the rest – US and other country’s point of view. (If I can. ) 1. US hold the hegemony of North East Asia. And the hegemony is highly dependent of US allies (Japan, ROK, and Taiwan?) in this region. I think US still can project its power over this region only if US keeps Guam its soil, but this is a minimum requirement – and as you know, minimum requirement means far away of satisfaction. 2. The growing power of China is a big threat to US allies and a challenge of the hegemony of US. China want to grab the hegemony and eventually bring US allies to her knee. 3. From 1 and 2 US and allies needs to make alliance even stronger. 4. ROK doesn’t want Korean war II (Thanks SGTOvious) and (maybe) Japan doesn’t - DPRK would attack US force in . And Japanese coast would be crowded by boat people. North Korean people in Japan would make noisy matters. Highly linked economy between Japan and ROK……. 5. US economy linked to this region could be seriously damaged by the WAR. US Investment on ROK and Japanese companies (Samsung, Toyota…), factories built by Korean companies on US, ROK is one of the biggest buyer of US weapons, and most importantly big jump of the price of PC would damage you guys…:) (ROK is one of the biggest producer of DRAM, monitor(display), CD-R, HDD…) 6. Russia – one of the resident of this region – doesn’t want war. Russia needs invest on Siberia from ROK and Japan – NATO countries are busy with their business. Korean War II will blow up the money. 7. China doesn’t want war. China cannot stand sharing the border with pro-US country but they needs the investment from US and the allies, so China want to take DPRK matter by itself not giving the chance of making United ROK or stationing of US troops along its border. 8. US can use DPRK nuke matter as a card against China. Nuke is a matter of US, but it’s a bigger matter of China. China cannot stand against US now, so it make itself away from the noisy conflict with US.(look at the Iraqi and central Asia matters) but… Nuke matter grabs China’s ankle - Conflict or Submission. 9. US is at war now. From Iraqi condition, US cannot easily mobilize big force against another region. 10. From these things you can guess – Is it a good time for Korean war II now? 11. These are not confidence of NO-WAR. US still hold the key. There are just possibilities of not going to war .