Democrat Senator Jay Rockefeller said thousands had died and yet Iraq had never posed a grave or growing danger.
However, the report by the Iraq Survey Group said there was evidence that Saddam Hussein intended to resume a weapons programme.
UK Prime Minister Tony Blair said that while he now accepted that Iraq held no stockpiles of WMD ready to be deployed at the time of the invasion, the report showed that UN sanctions had not been working.
The coalition used allegations of Iraqi WMDs as a key reason for going to war.
Chief US weapons inspector Charles said in the report that Iraq had no stockpiles of biological, chemical or nuclear weapons before last year’s US-led invasion.
Iraq’s nuclear capability had decayed not grown since the 1991 war, he added.