Richard, 

How on earth can you be sure it would "be over before they could launch anything"?   

North Korea can turn Seoul (population 10 million) into a sea of fire in seconds and is quite capable of nuking any and all of our South Korean bases while it is doing so.  For those involved, our "preemptive war" would feel very, very real.  It would cause - not preempt - the deaths of hundreds of thousands of South Koreans and tens of thousands of American servicemen.

Is this really the best response that the US can come up with to the threats and postures of a bankrupt regime?