The outgoing Pentagon number three has admitted holding doubts over key areas of US military policy in Iraq.
Douglas Feith has been identified as a leading hawk in the Pentagon
In an interview with the Washington Post, Douglas Feith said too few troops may have been deployed to invade Iraq.
The US had also delayed the transfer of power to Iraqi officials, and had missed the chance to train Iraqi exiles before the war to help the US, he said.
Mr Feith's resignation was announced in January. Seen as a hawk, he has been a driving force behind US policy in Iraq.
'Pros and cons'
Speaking to the Washington Post, Mr Feith expressed doubts over whether the right balance was struck on the size of the invasion force in Iraq.
He said: "I am not asserting to you that I know that the answer is, we did it right. What I am saying is it's an extremely complex judgement to know whether the course that we chose with its pros and cons was more sensible."
The US military has faced criticism that its current 135,000-strong force in Iraq is too small to curb the insurgency.
Mr Feith said "course corrections" also had to be made because of policy misjudgements.
One was the reluctance of some US officials to transfer power early on to Iraqi officials, and to dismantle the Coalition Provisional Authority. Control was handed over in June 2004.
Mr Feith said some US officials had not wanted to rely on "externals", or Iraqi exiles, to run the government.
"My views were generally in favour of transferring responsibility to the Iraqis earlier. I thought there were ways of getting the 'internals' involved earlier," he said.
He also believed the US should have done more before the invasion to train Iraqis in exile to assist the US military.
The Pentagon said in January that Mr Feith, who oversaw the Office of Special Plans, was stepping down for personal reasons. Critics claim his office misrepresented intelligence on Iraq before the war.