Technically, North Korea fired a missile at Japan and this could have been a good enough reason but wasn't. North Korea could erase life in South Korea, at cost of its own desintegration, but this wouldn't be good to anyone. NK is climbing as higher as it can to get the best deal available, which probably is keep their nuke capacity and the lifting of the sanctions. Other than attacking, I don't know if the US has other option than accepting NK demands.