"On the big decisions of sending the troops in, I'd have done it again," Bush told a questioner after a speech in Irvine, California, on immigration on Monday.
He said the US misjudged the internal security threat and mistakenly attempted big reconstruction projects that became a target for anti-US fighters.
"The fundamental question on the Iraq theatre, though, is: Did we put enough troops in there in the first place? That's the debate in Washington," he said. "I'm sure you've heard about it."
He said he told now retired General Tommy Franks, who developed and executed the Iraq invasion plan, "You design the plan and you've got what you need."
Bush said a new democracy is arising in Iraq where there once was tyranny. Over the weekend, Bush talked with Iraqi leaders who were named on Saturday to form a coalition government.
"Each one of them said, 'We want to have a national unity government. We're sick of the sectarian violence. We believe if you stand with us, we can achieve our objective of becoming a democracy that listens to the people'," Bush said.
"And I believe them. And I believe them. And I told them, I said, 'Look, it's going to be up to you to make it work, but you can count on the United States of America, because we believe in liberty and the capacity of liberty to change lives and to change a neighbourhood for a more peaceful tomorrow'."
Bush talked to al-Maliki, the Iraqi
prime minister, over phone
Bush's comments were echoed by Condoleezza Rice, the secretary of state, who said Iraq's new leadership provides the best opportunity to rein in rival militia groups blamed for a furious surge in sectarian killings.
Jawad al-Maliki, the Shia politician selected as Iraq's prime minister on Saturday after months of political infighting, is the strongest political figure to emerge since the US-led invasion more than three years ago, she said.
"He comes to this as the strongest political figure really ever ... since the liberation of Iraq," Rice said. "He comes with both the imprimatur of the Iraqi people and ... the mandate to form a unified national unity government."
For his part, Donald Rumsfeld, the US defence secretary, in an interview with the Pentagon's television channel, whose primary audience is the military at home and abroad, called the breakthrough in selecting top political leaders in Baghdad a "thrilling accomplishment".
Rumsfeld said he still expects to be able to reduce the size of the US force in Iraq, but he provided no details and said it would depend on conditions, including further political progress and the state of the anti-government uprising.
Rumsfeld: Defeat in Afghanistan
and Iraq would embolden Iran
Rumsfeld said who believe that US efforts in Iraq and Afghanistan are too costly or are taking too long need to understand that "success in Afghanistan and success in Iraq is critical to containing the extreme impulses that we see emanating from Iran".
Rumsfeld did not allude to military options in the interview.
"The last thing Iran wants is to have successful regimes, representative systems, free people in Afghanistan and Iraq," he said.
"It is harmful to their view of their world, to their extreme view of the world.
Rumsfeld added: "So most people who suggest that the cost is too great or it's taking too long, and we should not stay the course, it seems to me you have to think what it would mean to Iran and how it would advance their cause, and their cause is a cause that is dangerous to the world."