The United States needs to keep any eye on North Korea and act if military action becomes necessary. But any military action should be taken only after risk/reward analysis. There is no way North Korea could match the military strength or resources of the US and its allies. NK should think twice before making threats or taking offensive action.
No, military action is the absolute last resort and is never a good option. It is best to use all diplomatic avenues first. Look at Jimmy Carter, for example. He was able to create peace in some of the most heated areas in the world. I feel it is imperative to try all peaceful options first.
No, the United States should not use military action against North Korea. North Korea is not directly threatening the United States. Therefore, the United States does not have a right to use military action unless it is necessary to keep the peace. Otherwise, the United States should try to help resolve the matter diplomatically.
Until North Korea takes actions rather than making threats against other countries, I do not believe that it is the concern of the United States. Unwanted military action has gotten us in trouble with Vietnam and other countries. So unless North Korea's threats and actions directly affect the United States, we should not get involved.