To answer the immediate (and Topic) "What happens if Kim Jong-un acts on his threats?"
Much like a band of crazies rushing into a Wal-Mart with katanas and pistols on a Sunday morning, they will do a considerable amount of damage and kill a large number of soft targets for a while, until the big boys show up.
When the fuel and food stop, so will the offensive. By that time the North Korean Army will be perhaps 30~50 miles into South Korea. Seoul will be heavily damaged, and the government will have displaced south, perhaps all the way to Pusan.
Like Pearl Harbor (and Singapore, and Manchuria, and Kuwait ...), there will be an initial success for North Korea, and then the bubble will burst. The biggest losers will be the South Korean pacifists.
Or maybe the biggest losers will be the North Korean government, who will run for China as quickly as Idi Amin ran for Saudi Arabia.
Because if they stay, the collapse of the North will result in a young man -- and many others -- hanging from the end of a rope.
The first rule for a dictator is:
Stay Alive.
The second rule of a dictator is:
Stay in Power.
The second rule should yield to the first.
Those who understood those rules lived, some very well:
Idi Amin.
The Shan of Iran.
Manuel Noriega.
Those who got the rules reversed suffered the consequences:
Sadaam Hussain.
Muammar Gaddafi.
Benito Mussolini.
Tis better to cut and run, than stay and be cut.
Anyway ... that's my opinion, and it ought to be yours.
{uh, Scout Platoon, 1/38 th Infantry "the Rock of the Marne", 2nd Infantry
Division, 1974~ 1975}
