A nearly 700-page study released by the US Army found that "in the euphoria of early 2003," US-based commanders prematurely believed their goals in Iraq had been reached and did not send enough troops to handle the occupation.
President George W Bush's statement on May 1st, 2003, that major combat operations were over reinforced that view, the study said.
It was written by Donald Wright and Col Timothy Reese of the Contemporary Operations Study Team at Fort Leavenworth, Kansas, who said that planners who requested more troops were ignored and that commanders in Baghdad were replaced without enough of a transition and lacked enough staff.
Gen William Wallace, commanding general of US Army Training and Doctrine Command, said in a foreword that it is no surprise that a report with these conclusions was written.
"One of the great and least understood qualities of the United States Army is its culture of introspection and
self-examination," he wrote.
The report said that the civilian and military planning for a post-Saddam Iraq was inadequate, and that the Army should have pushed the Joint Chiefs of Staff for better planning and preparation.
Retired military leaders, members of Congress, think tanks and others have already concluded that the occupation was understaffed.
At least 4,113 US military members have died in Iraq.
Hundreds of commanders and other soldiers and officials were interviewed for the report. The Army ordered the study to review what happened in the 18 months after the toppling of Saddam Hussein's regime. A report on the invasion was released earlier.
The report said that after Saddam¿s regime was removed from power, most commanders and units expected to transition to stability and support operations, similar to what was seen in Bosnia and Kosovo.
Commanders with the mindset that victory had already been achieved believed that a post-combat Iraq would require "only a limited commitment by the US military and would be relatively peaceful and short as Iraqis quickly assumed responsibility," the study said.
"Few commanders foresaw that full spectrum operations in Iraq would entail the simultaneous employment of offense, defense, stability, and support operations by units at all echelons of command to defeat new, vicious, and effective enemies," it added.
The report said the first Bush administration and its advisers had assumed incorrectly that the Saddam regime would collapse after the first Gulf War.
When Saddam was so quickly defeated in 2003, there was an absence of authority that led to widespread looting and violence, the report said. Soldiers initially had no plan to deal with that. The administration's decision to remove Saddam's followers entirely from power caused governmental services to collapse, "fostering a huge unemployment problem," it said.