North korea:  what should the us do?   stance: nothing.:
Nothing the US can do with China stood right behind North Korea, any action you take they can counter act.
You just have to hope the Chinese can talk some sense into them. After all, there isn't a more westernised communist nation on the earth than China.
Besides, sanctions only harm the innocent citizens, it does not do much to stop the leaders.
Maybe proving Japan with nuclear weapons would combat North Korea, and act as a buffer? Who knows.