Well the issue is that North Korea actually wants to reunify and take over the South. So backing off the pressure gives them the opportunity to entrench further, and slowly build their nuclear arsenal to the point where, if they do invade the South, the US won't be able to respond in fear of a nuclear attack on its mainland. Essentially the North would conduct a lightning strike, lodge itself around Seoul, and then threaten Los Angeles if we counter-strike. Will the US sacrifice 10-30 million on the mainland for SK? I've read plenty of anlysis suggesting this is their end game. So, do we stop them now, because if they develop their ICBM to efficiency, and develop an H-bomb, their confidence to resume the Korean war could reach action level.