There’s not that much that can be done about Seoul. It’d be flattened to the ground in a day. The commies have so much arty in range there wouldn’t be two rocks standing on top of each other within a day or two. But they couldn’t move in south. That border is one of the most well defended ones.
They have the fifth-largest standing army in the world, and their backed by the Chinese who have the largest standing army in the world. If they invade the south, they’ll take massive casualties...but keep on coming. The regime doesn’t care how many of it’s men are chewed up by mines.
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Oh really? Know how many troops are on the ground in Panmunjun? Fewer than 600 Battalion Scouts (Light Infantry). These troops are nothing but a speed bump and they know it.
Drive south from the DMZ some time, every half mile to mile (iirc)is a fall-back position (berm with a gap). Every bridge over those gaps is wired to come down. Dragons teeth litter the fields in front of the berms.
The entire northern part of South Korea is designed to be given up but at a cost.