In the run up to the 2003 Iraq war, Saddam Hussein told us that he did not have weapons of mass destruction, but we didn’t believe him. But it turns out that he was telling the truth. The trouble is that he had lied to us so many times before, how could we tell this time?
That in layman’s terms is the CIA conclusion of why the U.S. intelligence community thought Iraq had weapons of mass destruction in 2003 when it really didn’t.
The revelations come in a CIA document "Misreading Intentions: Iraq's Reaction to Inspection Created Picture of Deception" obtained by the National Security Archive  through “mandatory declassification review request.”
The document is a “remarkable CIA mea culpa,” Tom Blanton, executive director of the National Security Archive, wrote for Foreign Policy magazine .
Blanton singles out this paragraph as key to the document:
According to Mark Thompson on TIME’s military intelligence blog , a “mandatory declassification review request” is faster than your standard freedom of information request. The National Security Archives made its request in 2006.
Thompson's bottom line:
Unfortunately, the U.S. went to war based largely on that false intelligence. Four thousand, four hundred and eight-six U.S. troops, 318 allies and untold thousands of Iraqis died in the ensuing conflict.
Thompson calls the CIA document: “mea maxima culpa.”