The problem is that Seoul lies within artillery range of North Korea, and you can't really defend against that, because it will take time to destroy all the art

The problem is that Seoul lies within artillery range of North Korea, and you can't really defend against that, because it will take time to destroy all the artillery, and in the meantime Seoul is shelled to pieces. South Korea could easily invade North Korea and defeat it, but the civilian casualties would be enormous unless you can figure out a way to protect Seoul. Complicating the matter is the fact that China is very pleased not having to share a border with South Korea, which they essentially view as an extension of the US. This was the whole point of the Korean war, after all.