Clinton cited the lack of planning for what would happen after Saddam Hussein was overthrown.
''Saddam is gone. It's a good thing, but I don't agree with what was done,'' Clinton told students at a forum at the American University of Dubai.
''It was a big mistake. The American government made several errors . . . one of which is how easy it would be to get rid of Saddam and how hard it would be to unite the country.''
Clinton's remarks came when he was taking questions about the U.S. invasion, which began in 2003. His response drew cheers and a standing ovation.
Clinton said the United States had done some good things in Iraq: the removal of Saddam, the ratification of a new constitution and the holding of parliamentary elections.
''The mistake that they made is that when they kicked out Saddam, they decided to dismantle the whole authority structure of Iraq. . . . We never sent enough troops and didn't have enough troops to control or seal the borders,'' Clinton said.
As the borders were unsealed, ''the terrorists came in,'' he said.
Clinton said it would have been better if the United States had left Iraq's ''fundamental military and social and police structure intact.''