Good job. They built a 5 kiloton nuclear warhead.
That's like half a MOAB.
North Korea has been testing missiles in the region for the past 30 years. Usually they drop somewhere in the Pacific with us knowing more about the actual performance of their rocket than they ended up with (because they don't have the force projection or technological capability to track high-trajectory missiles over such long distances).
The real threat North Korea poses is to South Korea. The RoK is the single manufacturer of the triple-hull tanker designs used for transporting oil, natural gas, and other liquids of commercial value. Their ports are vital not just for ship-building services, but also as cargo hubs for the rest of the world.
That's the real concern regarding North Korea.
They may just be capable of slop-rigging up a ballistic missile capable of delivering a nuclear payload equaling a few kilotons to the west coast... and bat-shit insane enough to do it. ... But that's only going to end with us being pissed and ROWFL-Stomping them. All we'll have to do is feed their soldiers and offer them croks... and that will win hearts and minds enough to get them to stop shooting. Popaganda efforts will be super-effective with the aid of South Korea and the ease with which we can collapse the house of cards the current regime stands upon. Basically, every North Korean is raised to believe that the rest of the world is jealous of Korea and that it is through the great and superior leadership people like Kim Jong Il and Kim Il Sun that the rest of the world is afraid to attack.
But it's possible that there could be Neodymium in them thar hills (as well as other rare earth elements - largely dominated by China, currently). That would greatly bolster South Korea's economy by giving them access to a natural resource in high demand but monopolized supply.
So there would actually be something worth mounting an invasion over. Aside from just getting revenge on pissing us off.