"It is true that much of the intelligence turned out to be wrong," Mr Bush said. "As president, I'm responsible for the decision to go into Iraq."
The president's mea culpa was accompanied by a robust defence of the divisive war.
"Saddam was a threat - and the American people and the world is better off because he is no longer in power," Mr Bush declared, as he has before.
Democrats were not moved by Mr Bush's speech, the last of four designed to boost his credibility on the war and the public's backing for it.
"There was no reason for America to go to war when we did, the way we did, and for the false reasons we were given," said Democratic Senator Edward Kennedy.
Mr Bush offered few qualms about the failure to find any weapons of
mass destruction in Iraq.
He said foreign intelligence agencies - including several for governments who didn't back his decision to invade - also believed before the war that Saddam Hussein possessed them.
And he said his administration has begun making changes to the US intelligence apparatus to head off future errors.
The president also contended the Iraqi president had intended to restart weapons programs.
No regrets
As in the past, Mr Bush acknowledged no regrets about launching the war despite the problems with his initial justification.
He revisited a long list of other previously cited reasons, including Iraqi violations of a no-fly zone in its airspace, Saddam's invasion of Kuwait a decade earlier and Iraq's defiance of United Nations resolutions.
"My decision to remove Saddam Hussein was the right decision," the president said to polite applause from his audience at the Woodrow Wilson International Centre for Scholars, a nonpartisan forum for the study of world affairs.
Mr Bush has repeatedly noted that the decision to go to war was his responsibility.
He has also acknowledged for more than a year that most of the intelligence behind the claims of Saddam's weapons programs turned out to be faulty.
But he has never linked the two so clearly and so personally.
On the eve of parliamentary elections in Iraq, Mr Bush's speech was meant to wrap up an aggressive push-back against war critics with an overarching explanation, nearly three years later, of why he went into Iraq and why he believes US troops must remain there.
The President’s remarks followed a pattern of more frank talk from Mr Bush on Iraq.
Voters split
Each installment in the recent round of Iraq speeches, which began last month at the Naval Academy, has included descriptions of fixes for early mistakes and sober assessments of remaining challenges.
That reflects the majority of Americans who, confronted with daily doses of bad news and rising death counts in Iraq, disapprove of Mr Bush's policies there and question the outlook for victory.
For instance, a new poll by the Pew Research Centre for the People & the Press found that most people see progress in areas such as establishing democracy and training Iraqi security forces.
But they are split on whether the United States is defeating the insurgents.
Answering critics who have said he's offered no definition of victory in Iraq, Mr Bush offered a succinct summation.
"Victory will be achieved by meeting certain objectives: when the terrorists and Saddamists can no longer threaten Iraq's democracy, when the Iraqi security forces can protect their own people and when Iraq is not a safe haven for terrorists to plot attacks against our country," he said.
"These objectives, not timetables set by politicians in Washington, will drive our force levels in Iraq."