it will fail again
There should be no mystery about why this is happening. All countries object to being occupied. Foreign invasions provoke nationalist resistance. This has happened with extraordinary speed in Iraq because of the ineptitude of the U.S. civil and military commanders, but in the long term it would have happened anyway.
The United States in Iraq has always behaved as if foreign powers or adherents of Saddam Hussein fomented the resistance. A lesson of the ground war last year was that few Iraqis were prepared to get killed for their old leader. Earlier this year I asked U.S. helicopter pilots operating from a base near Fallujah whom they thought they were fighting. They said firmly that they were at war with "FFs" and "FRLs." These turned out to be foreign fighters and former regime loyalists. One of the pilots added nervously that there seemed to be a third somewhat shadowy group "who want us to go home."