North Korea enjoys using harsh rhetoric against the United States and its allies, but the country is relatively weak and crippled. It employs a large army, but North Korea would never attack the United States and could be repelled by other nations anyways. The U.S. needs to impose harsher sanctions on N. Korea and show the country that there is no room for idle threats.
No, North Korea does not pose a threat to our national security. I believe that North Korea does not pose a threat because historically they have only threatened but never followed through on their threats when the time came. They have implicitly never "picked a fight" they were not sure they could win. In many ways they seem to pose more of a threat to countries smaller in size then their own.