The North Koreans may or may not have an actual functional nuclear bomb.

What they don't have is a miniaturized nuclear device that can go on a missile. That took the US and the USSR more then a decade after development of the weapon.

It's saber rattling by an extremely unstable regime who has their backs against the wall and an unknown as a leader.

Should they actually make any military moves, it's going to result in a lot of casualties and a nonexistent country. China, their sole ally, isn't happy with their actions, and may not back them in a military action, as China's economy is mostly reliant on trade with the US.

Their military looks impressive on paper. However, just ask Saddam how well starving troops fight. IN addition, they don't have the tech to stand up to a modern military.