North Korea has a ton of artillery at the DMZ, all either in defilade or in protected/tunneled positions.
Despite what the armchair generals here say, this is an intractable problem to deal with. If the U.S. pre-emptively attacks N. Korea and millions (or even thousands) in Seoul die or are injured as a result, it is unlikely those in charge will survive the backlash.
Only way to do this is to somehow neutralize the artillery at the DMZ or stage a successful coup that takes out Rocket Man.
Surely the US has polished a strategy over nearly half a century for taking them out quickly and efficiently.