North Korea does not have a deliverable nuclear weapon. And North Korea has nothing that can reach American Mainland at all.
Here are the two major threats posed by N Korea:
1. They have thousands of artillery tubes pointed towards Seoul. In case of any attack, Seoul will take heavy initial damage even if the attack capability is destroyed as quickly as technologically possible by USA.
2. N Korea has medium range tested missiles which can hit Japan. Japan will take a medium size hit during the time the artillery is being destroyed by the American air force and battle ships.
Apart from this, there is other major reason which is not tactical but political. N Korea is in China's backyard. Any show of power by America will carry a political statement and China will be forced to make its own political statement to create a balance.
No one knows what that action could be, and it is a big uncertainty. China can force Americans to relinquish their bases in Central Asia. Russia will definitely support, it is near their home too.
Even at this moment America is keenly aware of the dangers N Korea poses to it, if it acquires necessary technology to attack America. So, America is giving ample opportunity to Russia and China to solve this problem.
Basically America is saying this. "Look guys, we know it is your part of the world, so you take care of the problem. You pose to be big boys, now show it. However, if you are not able to control the pests in your backyard and they begin to pose danger to our home, we will come and destroy them ourselves."
China and Russia are hearing it loud and clear. They will do something if they are capable of. Remember, it is sheer media hype that China is some kind of superpower or Russia is a super power. Both are nothing in comparison to America. They may just not have the capability or political strength to neutralize N Korea. In that case America will wait till the last moment. Make China get red in the face while N Korea keeps slapping it silly. Japan will be clammering for intervention and increase its own military strength, which America will like very much.
In the end, America will neutralize N Korea and demonstrate to Japan, Australia and Central Asia that China is just a hype, real power still remains with America.
On the other hand, if China decides on a bold intervention, it will be mired in foreign disputes. It will taste the bitter fruit of being a super power which is forced to solve other people's dispute. It will invite the same kind of jealousy and hate which America invites these days.
It is all win win for America, always has been.