In theory they could strike Hawaii, but in reality the US along with most other countries in the world would smack their shit so hard they'd be back to the stone age before they got the chance (sadly for the average North Korean citizen, this isn't a huge change). The USSR and the US never attacked each other because they both understood the threat of mutual destruction. In this case, there is no threat of mutual destruction. North Korea would be obliterated and we would pretty much be fine.