“The Divine One shall exercise a mighty restraint..”
Overnight the Treasury Bond Market will evaporate. The USA will be dead in the water.
They don't have that many nukes. What nukes they have are low-power. If they hit us with nukes, then we shall retaliate with nukes.
Seoul is built up considerably from 1950. It is a megalopolis, with suburbs running almost all the way to the DMZ. Infantry advancing in human waves will be cut down by artillery and aircraft, as the Iranians learned in the 1980-1988 war. To advance, you need artillery and tanks - combined arms fighting. Seoul will be a defender's paradise.
The South Korean Army is very good and very professional. The Viet Cong feared them more than the US Army. The Korean Military Academy turns out first rate officers.
The North would run into a buzzsaw and would be mauled.