As planning for the invasion went forward, the Departments of Defense and State produced remarkably similar assessments of what could go wrong. In October 2002, Secretary Rumsfeld and his aides wrote a "Parade of Horribles" memo discussing 29 possible catastrophes. In retrospect, the memo proved remarkably prescient. Number thirteen was not finding weapons of mass destruction...
In mid- December, Secretary Powell received a twelve-page warning—co-authored by Ryan Crocker, eventual Ambassador to Iraq—titled "The Perfect Storm." This memo presciently warned that the struggle for dominance after the fall of Saddam would likely inspire violent clashes between and among Iraq's sects, tribes, and ethnic factions, possibly leading to the country's fragmentation.
Neither the "Parade of Horribles" nor "The Perfect Storm" memos were shared with the National Security Council's Executive Steering Group on Iraq and neither Rumsfeld nor Powell summarized the concerns they raised for officials working on day-to-day Iraq planning
Hard Lessons: The Iraq Reconstruction Experience The draft of a federal report by the Office of the Special Inspector General for Iraq Reconstruction. Annotations are based on the review's findings. The draft was provided to reporters at The New York Times and ProPublica by two people outside the Inspector General's office who have read the draft.
The Office of the Special Inspector General for Iraq Reconstruction (SIGIR) is the successor to the Coalition Provisional Authority Office of Inspector General (CPA-IG). SIGIR was created in October 2004 by a congressional amendment to Public Law 108-106 (55KB PDF), triggered by the June 28, 2004, dissolution of the CPA.
You can read online the 508-page draft report here.