North Korea is threatening civilian aircraft flying near its airspace. The excuse is the military exercises that South Korea and the U.S. are conducting. Civilian aircraft are being re-routed further from the edge of the border.
This is just posturing by North Korea. However, it could be fatal to people traveling in any aircraft in the area, so re-routing them is a good idea. It would not surprise me if they did shoot down a passenger airplane, and I doubt our response would be more than a diplomatic protest. In order to prevent the re-routing from becoming permanent or requiring a constant fighter escort for any civilian traffic, I would make a demonstration near the borders in question.
I would send a few groups of naval aircraft near the border to make the point that it IS international airspace. If they want to shoot at fighter aircraft, they can go ahead. At least our pilots have accepted the risk and are well equipped to defend themselves. I doubt that North Korea would want an air battle as the likelihood of a political black eye by having a bunch of their aircraft shot down would be more than they are likely to risk.
Not that you want to start a war. However, in order to prevent adventures in the future that are far more likely to result in a more general war, it is necessary to make a firm stand. When dealing with bullies, as this incident with threats to civilian aircraft demonstrate, it is far better to face them straight away and let them know where the line is. Otherwise, you can be easily pushed into a position where you must fight with everything that you have, instead of just sparring.