There is no easy answer.  The army has scenarios of a preemptive strike by the US and the scenarios all end badly because of the ability of NK to retaliate and fire quickly before we could destroy all of their firepower. South Korea and our troops would suffer heavy damage and deaths even if South Korea never fired large missiles or used nuclear weapons. So we need China to apply pressure by cuttting off trade to and from NK. But will that bring them to the table? Then once we get them to the table how can we get them to a realistic agreement that satisfies NK, US, and China? Tough one.