Machael DeLong, from the New York Times:
This is why the much-repeated claims that Mr. Rumsfeld didn’t “give us enough troops” in Iraq ring hollow. First, such criticisms ignore that the agreed-upon plan was for a lightning operation into Baghdad. In addition, logistically it would have been well nigh impossible to bring many more soldiers through the bottleneck in Kuwait. And doing so would have carried its own risk: you cannot sustain a fighting force of 300,000 or 500,000 men for long, and it would have left us with few reserves, putting our troops at risk in other parts of the world. Given our plan, we thought we had the right number of troops to accomplish our mission.
The outcome and ramifications of a war, however, are impossible to predict. Saddam Hussein had twice opened his jails, flooding the streets with criminals. The Iraqi police walked out of their uniforms in the face of the invasion, compounding domestic chaos. We did not expect these developments.
Here.