A series of secret U.S. war games in 1999 showed that an invasion and post-war administration of Iraq would require 400,000 troops, nearly three times the number there now.
And even then, the games showed, the country still had a chance of dissolving into chaos.
In the simulation, called Desert Crossing, 70 military, diplomatic and intelligence participants concluded the high troop levels would be needed to keep order, seal borders and take care of other security needs.
The documents came to light Saturday through a Freedom of Information Act request by George Washington University's National Security Archive, an independent research institute and library.
"The conventional wisdom is the U.S. mistake in Iraq was not enough troops," said Thomas Blanton, the archive's director. "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 about 144,000 U.S. troops in Iraq, down from a peak in January of about 160,000.
A week after the invasion, in March 2003, the Pentagon said there were 250,000 U.S. ground force troops inside Iraq, along with 40,000 coalition force troops.
A spokeswoman for the U.S. Central Command, which sponsored the seminar and declassified the secret report in 2004, declined to comment Saturday because she was not familiar with the documents.
News of the war games results comes a day before judges are expected to deliver a verdict in Saddam Hussein war crimes trial.
The war games looked at "worst case" and "most likely" scenarios after a war that removed then-Iraqi President Saddam Hussein from power. Some of the conclusions are similar to what actually occurred after the U.S.-led invasion of Iraq in 2003:
"A change in regimes does not guarantee stability," the 1999 seminar briefings said. "A number of factors including aggressive neighbors, fragmentation along religious and/or ethnic lines, and chaos created by rival forces bidding for power could adversely affect regional stability."
"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."
"Iran's anti-Americanism could be enflamed by a U.S.-led intervention in Iraq," the briefings read. "The influx of U.S. and other western forces into Iraq would exacerbate worries in Tehran, as would the installation of a pro-western government in Baghdad."
"The debate on post-Saddam Iraq also reveals the paucity of information about the potential and capabilities of the external Iraqi opposition groups. The lack of intelligence concerning their roles hampers U.S. policy development."
"Also, some participants believe that no Arab government will welcome the kind of lengthy U.S. presence that would be required to install and sustain a democratic government."
"A long-term, large-scale military intervention may be at odds with many coalition partners."