Well as we saw from WW2 and how Britain had failed to do anything to prevent Hitler's rise I don't think it would be a good idea to sit back and do nothing. We let Hitler rearm and gain territory and sitting back and letting North Korea expand their nuclear capabilities is not a great thing.

We don't know for sure if they do have a warhead capable of striking the US but North Korea will always continue to flex their military muscle leading to constant retaliation by America drastically increasing tensions.

In the end if the order is given, North Korea will be the first one to strike but in the end it may lead to their demise but that is if China and Russia do not get in the way.

At least North Korea will probably not be able to make anything as advanced as the Russian missiles which can constantly change flight path to prevent it being taken down by an antiballistic missile so if NK were to launch anything a SAM should be able to deal with it.