The United States cannot right the world’s wrongs. For the most part (there are exceptions), we will have to step in when those wrongs influence other nations though.
So when North Korea threatens to attack South Korea or Japan, that is a problem. Especially when we agreed to protect both countries from attack by treaty. They exist in their present form because of the US. So when Obama, Bush, and Clinton do an ostrich in the sand to North Korea, that is a big problem.
It is the responsibility of the United States to protect both countries because we have a treaty that says that we will. As long as the Kim family is only cruel to their own people, then we can leave them alone. When the Kim family and the leading generals of North Korea threaten the safety of Japan and South Korea, then it is equivalent to threatening the US.