Washington has for decades tried to convince North Korea not to pursue nuclear weapons, and then once it had them, to give them up. 

This hasn’t worked, for the very simple reason that there is no better security guarantee for a paranoid Pyongyang leadership that understands much of the world would rather it be gone. However, there have been times when it looked like U.S. policies were making inroads — sometimes to the surprise of long-time North Korea watchers.