Given your exchange with Dread, I'm assuming you haven't read tactical analyses of potential SK-NK confrontations. They aren't pretty.
Absent American intervention, SK is extraordinarily outnumbered and while technologically superior, has a limited ability to protect Seoul in the event of a war. NK has about 1.1
million active duty troops; SK has just over half that. NK also has a ridiculous number of long range artillery pieces (I saw some estimates of 10k pieces, most protected from airstrikes and close to the DMZ), a number of which can reach Seoul without moving an inch. NK has ridiculously extensive tunneling networks and experience, which would make comprehensively clearing an area difficult to impossible, and may allow them to launch an initial invasion from further south of the 38th parallel. While their air defense system is not state of the art, it's still pretty extensive and would give an unaided SK air force serious trouble establishing air supremacy. On the naval side, the South Koreans have a lot of technological superiority, but asymmetric threats (such as microsubs and the like) such as the ones that sank the Cheonan are likely to be a deciding factor that will keep the SK Navy largely out of the conflict. Let's also not forget they have some nukes.
Could the DPRK military beat the ROK forces in detail? Probably not; ROK planners have been working on this contingency for a long time. The active military is set up in protected forward positions to absorb the initial assault while the reserves get mobilized, and their advantage in the air would help blunt the rather large armor force that would be rolling at them. But their plans would likely leave hundreds of thousands (if not millions) or South Korean civilians dead, are not guaranteed to protect Seoul from being overrun, and largely necessitate a long, drawn-out confrontation.