[JediKooter]

North Korea can not stay in a long haul fight. Anything they do would have to be quick and decisive. I would be surprised if they had enough fuel for their tanks and other vehicles for more than a month, unless that's why China has been buying so much oil these last few years and is stockpilling it for North Korea. Besides, this is the wrong time of year for them to attempt an invasion of South Korea. They'd have to do it when everything is frozen so the tanks and troops that don't sneak through the tunnels can make it over land and not get bogged down.

You have a huge Army base in Seoul. A large air base in Osan, another air base in Kunsan. Then there's Pusan for the Navy and of course all the assets in Japan as well. North Korea would be stupid to attempt anything unless they are backed by Chinese assurances of assistance, be it military or financial or both.

I think with any dictatorship type of regime and it's leader is getting close to either dying or stepping down, you will have a few 'rouges' that try to position themselves as either viable replacements or to show the new regime that they are not weak. So, this dude that was playing Hunt for the Red October sank a South Korean ship, knowing that they are just going to get a harshly worded letter in the end and he shows any new potential bosses not to fuck with him.

That's just my 2 cents.