A panel headed by former National Security Advisors Brent Scowcroft (Ford & GHWBush admins) and Sandy Berger (Clinton) has completed its report on Bush's post-war planning for Iraq finding that the poor planning was a significant factor leading to the quagmire we now find ourselves in.
"A dramatic military victory has been overshadowed by chaos and bloodshed in the streets of Baghdad, difficulty in establishing security or providing essential services, and a deadly insurgency," the report said....
The report said the critical miscalculation of Iraq war-planning was the conclusion that reconstruction would not require more troops than the invasion itself.
A story about the report can be found here (remember to recommend it), and the report itself can be found on the Council on Foreign Relations website [220k PDF] - sponsors of the panel that prepared the report.