Simply put, before it is willing to negotiate with North Korea, the United States wants to completely, verifiably and irreversibly dismantle North Korea’s potential for the production of nuclear weapons.  However, in practical terms, extinguishing North Korea’s nuclear capability to the extent desired by the United States could take years to complete.  This seems to indicate that the current administration is content to wait out the current nuclear crisis and hope for a regime collapse rather than pursuing a policy of engagement with North Korea. (For more information on this subject, see http://www.nautilus.org/DPRKbriefingbook/intro.html)