+ they have a formidable army; 1+ million troops and plenty of powerful hardware including nukes
and they don't have any resources. Iraq is paying for humanitarian aid and will pay for rebuilding with their oil proceeds; Together with lucrative contracts for US companies for oil exploration, mobile telephone networks, power plants, ... the war could eventually be a boost to the US economy. After reunification, North Korea will be a burden on the South for decades to come, like East Germany is dragging down the west for more than 13 years now with no end in sight.