The New York Review of Books: Iraq: What Went Wrong:
By General Wesley K. Clark
"The decisive phase of the American campaign to invade Iraq and seize Baghdad was remarkably successful. Basically, the operation succeeded because of the competence of the fighting units, especially the men and women handling weapons and equipment. Without the edge their abilities gave us, the air forces could not have hit the Iraqi forces effectively on the ground; nor could the ground forces have advanced through the Republican Guards so quickly and with such light casualties. The US commanders' role was to shape that competence and apply it to the situation at hand. This they did brilliantly."

But there were also problems that should not be ignored. First, the military plan took unnecessary risks, because it skimped on the forces made available to the commanders. And while the level of forces proved adequate for defeating the Iraqi military, the central idea in military operations is effectiveness, not efficiency. Military operations should not be run like businesses, which have predictable requirements and aim to minimize the costs of meeting them. Combat, especially land combat, is one of the most unpredictable of human activities. It is inherently risky, with the risks usually resulting from factors that are improbable or cannot be foreseen. Therefore, sound logic dictates the need to minimize foreseeable dangers before beginning any military operations.

Additional forces were available— they were even under orders to prepare for combat in Iraq. One more combat division, an additional force for securing the supply lines, more trucks and supply units to provide the redundancy that the inherent inefficiency of military operations requires—each would have reduced the risks. Some of the planners knew this; whether these forces would be used was the issue at the heart of the continuing tensions during the planning process. But they weren't deployed until it was too late.

In early March, by the time it became clear that Turkey wasn't going to permit the passage of US troops, it was too late to position a fourth combat division in Kuwait. But at least the 2nd Cavalry Regiment or additional military police units could have been airlifted into Iraq. Instead their arrival was delayed until after the crisis along the supply lines five days after the war began. It is difficult to believe that additional forces along the supply route would not have helped to prevent the kind of mistake that cost some of the soldiers of 507th Maintenance Company their lives.

The war plan's excessive risk became clear in the postcombat stage, and here the US forces and capabilities were unequal to the task. It was the planners' job to have anticipated the various contingencies and to make adequate provision for them, including the possibility of postwar Iraqi resistance to US occupation. The "rolling start" philosophy of the top commanders, which seemed to emerge as much from continuing deployment problems as from any strategic calculus, made this impossible. The result, at the end of major combat, was a US force that was incapable of providing security, stopping the looting and sabotage, and establishing a credible presence throughout the country—even within Baghdad. The ensuing disorder vitiated some of the boost in US credibility that was won on the battlefield, and it opened the way for deeper and more organized resistance during the following weeks.…

http://www.nybooks.com/articles/16650