US senators have severely criticised the country's intelligence agencies, in particular the CIA, for the quality of their pre-war information on Iraq.
The report highlights multiple intelligence failures
In a scathing report, the Senate Intelligence Committee says the CIA overstated the threat posed by Iraq.
As a result, the US and its allies went to war based on "flawed" information.
However, the report concluded there was no evidence the Bush administration had tried to coerce or put pressure on officials to adapt their findings.
Most of the key judgements about Iraq's WMD programmes "were either overstated or were not supported by the raw intelligence reporting," said the committee's chairman, Republican Senator Pat Roberts.
The intelligence community suffered a "collective group-think", which led analysts to presume that Iraq had weapons of mass destruction (WMD) programmes and to interpret ambiguous evidence as conclusive, Senator Roberts said.
SENATE REPORT: KEY POINTS
Assumptions about Iraq wrong, not supported by evidence
Analysts failed to say when intelligence was uncertain
Managers failed to question analysts' assumptions
CIA had no human sources in Iraq since 1998
CIA withheld intelligence from other agencies
But the failings were not America's alone.
"It is clear that this group-think also extended to our allies, and to the United Nations, and several other nations as well, all of whom did believe that Saddam Hussein had active WMD programmes. This was a global intelligence failure," he said.
The report said there was no evidence that analysts came under pressure from the White House to deliver certain findings, although some Democrats dissented from this conclusion.
The issue of whether the Bush administration exaggerated the case for war in Iraq is being investigated separately in a report most likely to be released after the presidential election on 2 November.
The Democrat vice-chairman of the committee, Senator Jay Rockefeller, stressed his party's regret that the whole matter had not been addressed in one inquiry.
He said many members of Congress would not have authorised the war if they had known then what they knew now.
"Tragically, the intelligence failure set forth in this report will affect our national security for generations to come," he said.
"Our credibility is diminished; our standing in the world has never been lower. We have fostered a deep hatred of Americans in the Muslim world, and that will grow."
Responding to the report, President Bush promised to make sure the intelligence agencies were reformed.
Speaking on the campaign trail in Pennsylvania, he also stressed that the world knew Saddam Hussein had been trying to acquire nuclear, biological and chemical weapons.
"We haven't found the stockpiles, but we knew he could make them," Mr Bush said.
The US report comes just five days before Lord Butler publishes the results of his inquiry into the quality of British intelligence on Iraq.
CIA director George Tenet, who steps down on Sunday, was criticised for not personally checking President Bush's 2003 State of the Union address.
This contained the allegation - which first surfaced in a UK report and since discredited - that Iraq had been trying to buy uranium from Niger.
The deputy director, John McLaughlin, said people should not conclude from the Senate report that there were huge failings within the CIA.
He told reporters "it is wrong to exaggerate the flaws or leap to the judgment that our challenges with pre-war Iraq weapons intelligence are evidence of sweeping problems."