These North Korean facilities are housed inside mountains and mountains of rock are very effective defense structures (think of the American NORAD Command Bunker in Cheyenne Mountain).

"Well, they can only hit Alaska and Hawaii",  say the eternal optimists or that we can "liquidate" any incoming threat. There are decoy warheads which will be deployed to frustrate anti-missile defenses. Also, an attack might be launched from offshore using a readily available short range delivery system  or by other means including a shipping container.  Many seem to forget an additional dimension of thermonuclear munitions other than heat and blast - radiation.  Look at the prevailing jet stream that often curves over Alaska and downward into the lower 48 and keep telling yourself that they can only impact Alaska. 

Mutually Assured Destruction (MAD) is the only viable alternative between nation states armed with nukes and North Korea must be made to understand that concept in no uncertain terms. 
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