A prestigious US political research body has accused the US government of giving impetus to Iraq's insurgency through a lack of post-war planning.
Militant groups have carried out almost daily attacks in Iraq
A Council on Foreign Relations study said the decision that reconstruction would not need any more forces than the invasion was a critical miscalculation.
Too few soldiers, it said, had left the US ill-equipped to address security, governance and economic demands.
Post-war development must be a national security priority like war, it said.
The report also recommended that an international fund of $1bn be set up to help reconstruct post-conflict states.
It was headed by two former national security advisers - the Democrat Sandy Berger and the Republican Brent Scowcroft, who was critical of the invasion of Iraq.
The report said that government responsibility for stabilisation and reconstruction was "diffuse" and "uncertain".
"Nation-building is not just a humanitarian concern, but a critical national security priority that should be on par with war-fighting," it said.
"The failure to take this phase of conflict as seriously as initial combat operations has had serious consequences for the United States, not just in Iraq but, more broadly, for international efforts to stabilise and rebuild nations after conflict."
Specifically, it said the perceived failure to prepare properly for the post-war period had given an "early impetus for the insurgency".
Iraq's efforts at reconstruction have been hampered since the 2003 invasion by almost daily shootings and bomb attacks.
Militants have stepped up their attacks in recent weeks in a bid to destabilise Iraq's new Shia-led government.