The point is during the Gulf War, Bush insisted that Iraq had weapons of mass destruction and it was on this premise that the war was fought. After the war, there was none to be found.  Here the joke is that Kim says he has nuclear weapons but he really didn't have any.  So if Obama listens to Bush and calls his bluff.... Hence Kim said shit.

Joke. Hilarious.Funny. I could not understand the comedy behind this. Please explain

I found this picture in facebook . The comments were hilarious but I could not completely infer the hidden humor . May be because of my poor knowledge about Bush or Obamas . I know about the political war like situation in Korea and American connection to it but still nothing made me laugh.
Just help me understand this humor
Thanks in advance