Can you say airstrikes on the demilitarized zone as they come across? On the other hand, our enemies seem to like tunnels.
We do not have enough assets in theater to knock out one million men. We would kill 50,000 and that would leave 950,000 to overrun the south. It will be a battleground which harkens back to the tactic of the Chinese. Loosing hundreds of thousands of their men is meaningless to them. They are chattle to Kim.
If they move any significant numbers to those tunnels, we'll know where they are. Then it's bunker busters and thermobaric bombs on the entrances. Not a nice way to go.
The NK would get a flurry of smart bombs down their throats 24 /7.