First his artillery and mrls unload on the DMz and extending into Seoul
Then his commandos hit in the rear, as several million panicked civilians flee south , confusing the battle zone as his fanatically dedicated forces head south
Meanwhile there is a strategic launch out toward Japan which hits nothing vital but nonetheless raises the question
Do you hit back based on his effect ( minimal target damage if any) or his intent ( which may be unknown and hotly debated ...)
One problem with that scenario: fuel. From what I’ve been reading the North Koreans either could not mount a mechanized invasion or, if they could, sustain one. So what you have is the combination of a nuclear strike & a commando/guerrilla war. Decisive? Probably not, since the US would drop a tactical nuke on Pyonyang while the SK Army ruthlessly wipes out the Nork leg infantry.