The US invasion plan for Iraq envisaged that only 5,000 US troops would remain in Iraq by December 2006, declassified Central Command documents show.
The US is now sending some 20,000 extra troops to Iraq
The material also shows that the US military projected a stable, pro-US and democratic Iraq by that time.
The August 2002 material was obtained by the National Security Archive (NSA). Its officials said the plans were based on delusional assumptions.
The US currently has some 132,000 troops in the violence-torn state.
The documents - in the form of PowerPoint slides - were prepared by the now-retired Gen Tommy Franks and other top commanders at the time.
The documents were presented at a briefing in August 2002 - less than a year before the US-led invasion of Iraq in March 2003.
The commanders predicted that after the fighting was over there would be a two- to three-month "stabilisation" phase, followed by an 18- to 24-month "recovery" stage.
They projected that the US forces would be almost completely "re-deployed" out of Iraq at the end of the "transition" phase - within 45 months of invasion.
"Completely unrealistic assumptions about a post-Saddam Iraq permeate these war plans," NSA executive director Thomas Blanton said in a statement posted on the organisation's website.
"First, they assumed that a provisional government would be in place by 'D-Day', then that the Iraqis would stay in their garrisons and be reliable partners, and finally that the post-hostilities phase would be a matter of mere months'," Mr Blanton said.
"All of these were delusions," he added.
The NSA said it received the documents last month, after making a request in 2004.
The NSA is an independent research institute at George Washington University.
It obtained the papers under the Freedom of Information Act.