Kim Jong Un has put his country in a very difficult position. His propaganda machine may be talking some tough talk, but I doubt he can walk the walk. One, two, three nukes does NOT make a super-power. Moreover, he doesn't have an airforce worth a damn nor a Navy that can compete with the U.S. His only hope is to pull another country into his mess - China and/or the USSR - and hope he can get the rest of the world (well, the really anti-American radical side anyway) to side with him should the American government jump the gun and launch a pre-emptive strike. Problem there is any move NKOR makes that gives even the slightest appearance of hostilities towards SKOR, any U.S. territory or one of its allies will demand a swift response. In short, if anyone from NKOR as much as tosses a rock in the direction of SKOR there will be retaliation.
The key to keeping things relatively tame is China. NKOR knows it can't hope to defeat the U.S. on its own. It needs an ally. As long as China stays out of it, NKOR won't do a thing.