Why else would the North Koreas use a nuke? They aren’t stupid, they wouldn’t kill themselves by launching a nuclear weapon at anyone. Just becuase the cold war is over doesn’t mean the game of mutually assured destruction ended too. Therefore, the only reason for the Koreans to use such a weapon would be in self defense.
I don’t care how long our generals have been planning an invasion of Korea, one cannot prepare for a nuclear war. If you have a large force in the open (say, in the middle of an offensive) when a nuclear device is detonated then, chances are, everyone in that force is going to die.