US efforts to rebuild Iraq and train its armed forces have been costly
US reconstruction efforts in Iraq were "grossly overburdened" by wasteful spending, according to a report by the special inspector general for Iraq.
Stuart Bowen blames a lack of security and "dramatic" course changes for the waste, but says fraud was a "relatively small" problem.
His report comes as a congressional commission set up to monitor spending has been holding its first hearings.
The US has spent nearly $51bn (£36bn) on rebuilding Iraq and its army.
Officials initially estimated that the reconstruction programme would cost $2.4bn.
America "was neither prepared for nor able to respond quickly to the ever-changing demands" of the programme, said Mr Bowen, the Special Inspector General for Iraq Reconstruction, in testimony before the congressional commission on wartime contracting.
"For the last six years we have been on a steep learning curve," he added.
Mr Bowen began overseeing US expenditure on Iraq about five years ago, and his report is an attempt to set out the lessons he and his team have learned over that period.
According to the report, when he first took the job, then Defence Secretary Donald Rumsfeld asked him: "Why did you take this job? It is an impossible task."
On his first trip to Iraq, he says in the report, he observed large quantities of cash moving out of the office of the comptroller for the Coalition Provisional Authority, after which he overheard someone say: "We can't do that anymore. There is a new inspector general here."
Mr Bowen is concerned that the mistakes made in Iraq are being repeated in Afghanistan.
"It's too late to do the structural part and make it quickly applicable to Afghanistan," he told the Washington Post.
President Obama should avoid having "multiple versions" of federal acquisition regulations in Afghanistan by issuing a single set of regulations "that everyone will follow", Mr Bowen says.