The 70 military, diplomatic and intelligence officials who took part in "Desert Crossing" assumed that at least 400,000 troops would be needed to keep order, seal borders and take care of other security needs if they invaded Iraq to overthrow Saddam Hussein.
"The conventional wisdom is the US mistake in Iraq was not enough troops," Thomas Blanton, director of the George Washington University's National Security Archive, an independent research institute and library, said.
"But the Desert Crossing war game in 1999 suggests we would have ended up with a failed state even with 400,000 troops on the ground."
There are currently about 144,000 US troops deployed in Iraq, down from about 160,000 in January.
The study - which became public through a Freedom of Information Act request by the archive on Saturday - tested "worst case" and "most likely" scenarios for a post-Saddam Iraq.
Stability 'not guaranteed'
The state department, defence department, national security council and the CIA all took part.
A spokeswoman for US Central Command, which sponsored the study and declassified the report in 2004, declined to comment saying she was not familiar with the documents.
Several years before the sectarian violence which has developed since the 2003 invasion the 1999 report was able to warn that "a change in regimes does not guarantee stability."
"A number of factors including aggressive neighbours, fragmentation along religious and/or ethnic lines, and chaos created by rival forces bidding for power could adversely affect regional stability."
The report was similarly pessimistic about the kind of government which would follow Saddam.
"Even when civil order is restored and borders are secured, the replacement regime could be problematic - especially if perceived as weak, a puppet, or out-of-step with prevailing regional governments," it said.
"A change in regimes does not guarantee stability"
Desert Crossing After Action Report, 1999
A transitional government set up by the US would be likely to encounter difficulties "from a period of widespread bloodshed in which various factions seek to eliminate their enemies," contributors suggested.
The report also stressed that the creation of a democratic government was not feasible but a new pluralistic Iraqi government including nationalist leaders could be possible.
Marine General Antony Zinni, who lead Centcom at the time of Desert Crossing and retired short after its completion, has repeatedly said that many lessons that could have been learnt from the report were ignored.
Before the invasion, Zinni says he raised the issue of Desert Crossing but it had seemingly been forgotten by Central Command.
"When it looked like we were going in [to Iraq], I called back down to Centcom and said, 'You need to dust off Desert Crossing.' They said, 'What's that? Never heard of it.' So in a matter of a few years it was gone," he said in a speech in 2004.
The report said that the government and military should work towards a stable, unified country in the years following an invasion.
More than three years after George Bush declared that the mission had been completed in Iraq, his Republican party is under pressure over the situation ahead of congressional elections on November 7.
National Security Archive: http://www.gwu.edu/~nsarchiv/index.html