Once the North Korean army moves it will be annihilated by our Air Force, I mean total annihilation.
I've seen these wild extremes between people assuming the NKs get instanty annihilated by airpower in an easy war, and people under the rather silly impression the NKs would somehow roll forward a huge distance into South Korea in an unstoppable juggernaut (dumb on two levels - one, I honestly think it's people unaware of the existence of the ROK army, and two, if the DPRK ever thought they REMOTELY had a chance of success in such an attack, they would have tried it already).
The truth lies in-between. The terrain is rugged and the weather usually bad which impairs airpower, but I don't see a North Korean attack getting much of anywhere - the troop densities are very high and the ROK army is a far, far better force than it was in 1950. And GPS bombs are a true revolution in warfare the scope of which we haven't fully seen yet.
It would be a heck of a lot bloodier than either Gulf War and more difficult, but some of the extreme casualty estimates seem a bit silly (as they did back before GWI, which I was pointing out on the old GEnie BBS at the time - I was roundly ridiculed back in December before that war started for guessing the land war would last two weeks with less than 1,000 US dead).
I respectfully disagree. The North could put 2 million men on the southern border of North Korea, and we would not act until they fire or move on us. We would only use conventional means which would limit a ability to effect millions. In that theatre we would not use nuclear devices, unless they first used it on our soldiers, and maybe not until they attacked our homeland. That means conventional warfare.