Personally, I think North Korea would have momentum seeing as that they would start the war (SK is fine with it's starcraft). However, SK has a very powerful army, albeit, lesser in numbers, but with much more modern weaponry (North Korea needs to stop using rusty Soviet era vehicles), American troops to help, and then Japan (Not sure what they would do, but I think they would do something to help out SK). After the initial surge from North Korea was halted, I have a feeling a counter offensive would begin, the North would be pushed back, then that's where I'm not certain. I don't know where China stands on this. If they act like how they did in the Korean War, then they would attack, however, this would mean trade would be cut off from them seeing as that the US and it's "allies" would cut all ties with China, thus causing major economic damage, but China desperately wants the "buffer" North Korea offers with the west. Anyways, if China were not to help, North Korea would probably retreat to the border, set off it's nukes, then the leaders would try and escape, and if caught, trialed for war crimes like all enemies of the US that lose, and the war would be over. North Koreans could finally live knowing, "hey, maybe the world doesn't actually hate us and that people live beyond the oceans"