There must be considered four components in regards of rebuilding a country: political reconstruction, economic reconstruction, social reconstruction and most importantly provision of general security. Security is, as Diamond writes, the central pedestal that supports all else; because without security one cannot begin to reconstruct the economy, nor can you achieve political and/or social reconstruction. Therefore the error of not taking into consideration the provision of security as a top priority in reconstructing Iraq had a greater impact that would be thought.
Bush was advised to send said troops to fully achieve different tasks that were necessary. Diamond estimates that 300,000 troops might have been sufficient to accomplish that. The different kinds of troops that needed to be appointed were, among others: urban patrols, crowd control, civil reconstruction, peace maintenance and enforcement, control over the borders with Syria and Iran, etc.
The scarcity of troops and lack of resources available to the Coalition Provisional Authority (CPA), greatly affected the situation in Iraq. Additionally there was a serious lack of organization, as cops were rushed onto the job without adequate training nor equipment which let to multiple terrorists attacks against police cops. This problem quickly become more prominent leading to the point were officials could no longer move around the country.
After the assassination of the mos influential pro-U.S. cleric in Iraq's Shiite heartland, Sayyid Farqad al-Qizwini, by Sadr's organization; U.S. forces went to war with the Mahdi army. These confrontation not only did it fail to eliminate Sadr's forces, but did nothing to counter other Iraq's armed militias. The U.S. was forced to rely on cooperation of SCRIRI and Dawa militias to evict and defeat the Mahdi army which in consequence reduced the CPA's leverage over them.
This security blunder could have been avoided by following the advised given frequently to president Bush.
