OK, let’s play a war scenario. A little one, just for the “fun” of it. Let’s say that the DPRK is so crazy to actually fire a couple of medium-range missiles. One straight to Japan, one down south to their separate brothers. In this game we can assume that both missiles will hit without malfunctions, both with conventional warheads.
OK, think about Japan. They got a right-wing government, already on bad terms with China for a matter of little islands (with a lot of gas under), their premier got elected on a nationalist program and with the promise to overcome their post-WWII constitution with the creation of a full-fledged military instead of the JSDF. A missile hit, no matter where it comes from, is a perfect time to force both matters into reality with all the public opinion at their backs. By the way Japan could have full-functional nuclear weapons in few years if they want.
OK again, think about South Korea. They elected as their new president the daughter of their historic dictator and when it comes to national security all the koreans are ready to fight back, no matter the cost for their economy. With a missile hit somewhere in the country they will retaliate in any way available and, following this kind of effort, they will go armed to the teeth and that will put a twist in the direction of part of their economy. Even South Korea got all the tech needed to get their own nukes in matter of five years.
That leaves USA in the middle of nowhere. Its role of bening protector could be shredded to pieces if they cannot stop any missile from DPRK. A pity that’s quite hard to do. Even the best system available is not perfect. In Japan and in South Korea there are a lot of forces hostile to the role of the USA, forces that want to raise mountains of money from the weapon industry. Their weapon industries. Not the american ones. China too will not be that happy about it, having a nuclear-armed Japan and a similar nuclear-armed South Korea at its backdoor.
That said, it could be difficult to explain who decide to start an attack. DPRK? The lobbies in Japan? Their friends in South Korea? Anybody else?
This is just a game. This could be the same line that Mr. Kim is telling right now at the chinese government, looking in the eye some stud from their Sixth Department.