What will N.Korea "attack" the US with if the US attacks 1st?
What guidance system do they have for their missiles? How to their missiles re-enter from the earth's atmosphere?
I'll tell you the answers, they don't and have no idea. 
If you've been paying attention, N.Korea has NEVER hit a target, not even close. The safest place from a direct hit from N.Korea is Guam.