"We promised Iraq democratic government. We will deliver it," he said to loud applause from a joint House-Senate meeting.
Without getting into specifics of the growing controversy over pre-war intelligence, Blair suggested that history will forgive the toppling of Saddam's government even if it turns out that he and President Bush were wrong about Iraqi weapons of mass destruction.
To have hesitated "in the face of this menace when we should have given leadership ... that is something that history will not forgive," Blair said.
He vowed Britain would stand with the United States in the long fight against terrorism and for liberty.
This was supposed to be a kind of victory celebration, a way to honor Blair, the only major ally to stick by the U.S. in the war on Iraq, reports CBS News Correspondent Bob Fuss. But instead it comes at an embarrassing moment, when the White House has distanced itself from British intelligence reports on Iraqi efforts to buy uranium and both Blair and President Bush are under fire for the pre-war claims of Iraqi weapons that have yet to be found.
After the address, Blair headed to the White House for a meeting and joint news conference with Mr. Bush.
"Can we be sure that terrorists and weapons of mass destruction will join together?" Blair asked. "But to say one thing, if we are wrong, we will have destroyed a threat that at its least is responsible for inhumane carnage and suffering."
Blair arrived aboard his British Airways jet in early afternoon and went directly to Capitol Hill. It was the first leg of a seven-day tour that will also take him to Asia. He is the first British prime minister to address a joint meeting of Congress since Margaret Thatcher in 1985.
Blair entered the House chamber to a standing ovation of lawmakers, senior Bush administration officials and American military brass.
The prime minister wryly thanked his audience for a "warm and generous welcome that's more than I deserve," and, in a reference to his political troubles at home, added, "it's more than I'm used to, quite frankly."
Blair's speech also touched on the war on terrorism, the Middle East peace process, the need to eradicate poverty, disease and famine in Africa and the need to promote free trade.
"This terrorism will not be defeated without peace in the Middle East between Israel and Palesitne," he said.
In what appeared to be mild criticism of the Bush administration, Blair also said it was important to act in coalitions, not going it alone. "Let us start preferring a coalition and acting alone if we have to, not the other way around," he said.
And he called on lawmakers not to continue to bear grudges against European countries who opposed the war.
"They are our allies. And yours. So don't give up on Europe," he said.
"When we invade Afghanistan or Iraq, our responsibility does not end with military victory," Blair said. "Finishing the fighting is not finishing the job. We promised Iraq democratic government. We will deliver it.
"We promised them the chance to use their oil wells to build prosperity for all their citizens, not a corrupt elite. We will stay with these people so in need of help until the job is done."
"I believe with every fiber of instinct and conviction I have that we are" right in deciding to go to war without broad international support, Blair said.
Ahead of Blair's visit, White House spokesman Scott McClellan reiterated the recent administration stance that Mr. Bush's mention of the British Iraq-Africa report should not have been included in the January address.
Still, he added, "the British have been very clear that they stand by that statement."
Mr. Bush said in his State of the Union address, "The British government has learned that Saddam Hussein recently sought significant quantities of uranium from Africa."
CIA Director George Tenet has thus far taken the blame for allowing the statement into the president's speech, suggesting he should have objected when a draft was circulated to his agency.
The White House tried to deflect responsibility away from the president — even though a page on the White House Web site shows a picture of Mr. Bush working on his speech, with a caption that says "President Bush reviews the State of the Union address line-by-line and word-by-word."
Asked about that on Thursday, McClellan said there were "a lot of people involved "who had input into the president's speech ... And the bottom line is the speech was cleared. But we learned some more information later and we should not have included it in there."
U.S.-British ties have also been strained by the fate of two British terror suspects being held at Guantanamo Bay.
Blair is under pressure to raise the issue of Moazzam Begg, 35, and Feroz Abbasi, 23 – Britons being held at the U.S. naval base in Cuba. More than 200 British lawmakers signed a parliamentary motion protesting any American plans to try them before a military tribunal.