I served in the 25th ID for four and a half years. Our main mission was North Korea. That certainly doesn't make me an expert, by any means, but my learned perspective might serve to add to our overall understanding.
In the late 90's the battle plan for the 25,000 or so troops in SK was a hasty retreat until the 25th could get there, followed by the 82nd & 101st. I realize this was some time ago, but I suspect the general strategy hasn't changed much. The DPRK certainly hasn't. They aren't dangerous due to nukes or technology. They are dangerous due to sheer numbers. I forget how many DPRK artillery pieces range Seoul, but it's in the thousands. That is, the opening moves would see artillery raining down on Seoul - not good. The DPRKs military technology is somewhere in the mid 1960's. However, there's a lot of it and what we have in place can't stop an invasion (initially).
I guess the "good" news is that any full court press invasion would require a massive build-up of personnel and equipment. On the other hand, if that portly dictator is willing to simply sacrifice a certain number of troops (never set up for success, but rather to make a horribly awful statement) I could see things kicking off with little to no warning.
I think the real danger here is Kim Jong Un (spelling?) is an utter amateur. There's a real possibility he'll blunder the DPRK into a war with South Korea, and by obvious extension, the U.S.
Scary stuff.
