"The war, I know, split the world," Blair said. "The struggle of Iraqis for democracy should unite it."
The prime minister was a rare European ally of President Bush in going to war — and like Bush he has suffered political consequences. Large majorities in both countries now say they disapprove of the way each man has handled his job and the Iraq war.
Blair met with Mr. Bush over two days this week, following a visit to Baghdad to meet with Iraq's new political leaders. The United States and Britain hope formation of Iraq's first permanent democratic government will improve security and build confidence, speeding the time when their combined 140,000 troops can leave.
CBS News has learned the Pentagon is sending in two new battalions, 1,500 additional soldiers, to some of Iraq's most troubled areas.
During his 22-hour visit to Washington, Blair and President Bush both adhered to their strict policy: no timetables on troop drawdowns, reports CBS News White House correspondent Jim Axelrod.
"We can argue forever about the merits of removing Saddam," but that is not the point, Blair said in the speech at Georgetown University.
"You may not agree with the original decision. You may believe mistakes have been made. You may even think, 'How can it be worth the sacrifice?"' Blair said. "But surely we must all accept this is a genuine attempt to run the race of liberty."
Blair said he was impressed with Iraq's multiethnic leaders. As long as they are willing to fight terrorism and violence, they should have the world's support, he said.
"These weren't stooges or placemen. They believe in their country. They believe in its capacity to be democratic. They are fighting against the odds, it is true, but they are fighting."
Blair argued, as Mr. Bush does, that a democratic bulwark in Iraq can help change the Middle East. He said little about Iran, which represents another brewing problem in the region that has also divided world opinion.
"I don't believe we will be secure unless Iran changes," Blair said. "I emphasize, I am not saying we should impose change."
Britain has again sided with the United States as it presses for tough international action to contain Iran's nuclear program, but both nations say the idea of another invasion or attack is far-fetched.
The United Nations Security Council is reviewing Iran's disputed nuclear activities, but the council's veto-holding members have been divided over whether to seek sanctions or other measures enforceable by military action.
At a White House news conference Thursday, Mr. Bush and Blair were defensive when they would have preferred to celebrate the recent political success in Baghdad.
Mr. Bush acknowledged the bloodshed has been difficult for the world to understand. Blair called the violence "ghastly."
But, Mr. Bush said at the White House, "Despite setbacks and missteps, I strongly believe we did and are doing the right thing."
Those missteps include the abuse of Iraqi prisoners at Abu Ghraib, though Mr. Bush said those responsible have been jailed. More personally, the president said, he learned not to use so much "tough talk," saying Osama bin Laden was wanted "dead or alive" and challenging America's enemies to "bring it on." This was an "extraordinary" admission from a president who is not normally known for public self-analysis, CBS News senior White House correspondent Bill Plante reports.
"I learned some lessons about expressing myself maybe in a little more sophisticated manner, you know," Mr. Bush said softly.