The Bush administration has kicked off an offensive to bolster support for America's role in Iraq exactly six months since the fall of the Saddam Hussein regime.
Condoleezza Rice: Saddam lied "right up until the end"
US National Security Adviser Condoleezza Rice said on Wednesday new evidence uncovered by weapons inspectors proved Iraq had posed a serious threat and that the invasion was justified.
President George W Bush will also make a keynote speech later on Thursday - set to be one of a string of speeches by senior government members aimed at defending the decision to go to war.
But the BBC's Rob Watson in Washington says the Bush line is still proving a hard sell at home and abroad, as US forces in Iraq continue to come under daily attack and suffer mounting casualties.
Dr Rice said the credibility of the United Nations would have been "in tatters" if the US had not acted against Iraq.
US-led weapons experts who have scoured Iraq since the war were "finding proof that Iraq never disarmed and never complied with US inspectors," she said.
"Right up until the end, Saddam lied to the Security
Council," she said in her speech to the Chicago Council on Foreign Relations.
"And let there be no mistake, right up to the end, Saddam Hussein continued to harbour ambitions to threaten the world with weapons of mass destruction and to hide his
illegal weapons activity", she said.
Dr Rice said the report by the Iraq Survey Group, led by former UN weapons inspector David Kay and presented to US lawmakers last week, provided "hard evidence of
facts that no-one should ever have doubted".
Critics have complained that the $300m search has failed to turn up any actual weapons of mass destruction in Iraq, but Ms Rice urged the American public to be patient.
Life remains volatile for the people of Iraq
She said that given more time the Iraq Survey Group will "put together the true story of what has happened, what came of Saddam Hussein's weapons of mass destruction".
With political support for America's role in Iraq on the wane Ms Rice cautioned that establishing democracy in Iraq would be a slow process:
"We must remain patient. Our own history should remind us that the union of democratic principle and practice is always a work in progress."
Dr Rice's speech was the opening salvo in a White House counterattack against opponents of Mr Bush's Iraq policy.
Mr Bush will defend that policy himself in a speech on Thursday, which correspondents predict will focus on what progress has been achieved in Iraq.
On Friday, it will be the US vice president's turn when Dick Cheney is expected to also turn his attention to attacking Iraq-policy critics in a speech in Washington.
The campaign comes as the US Congress begins to debate Mr Bush's demand for an extra $87bn in emergency funding to rebuild Iraq and Afghanistan.
The BBC's Jill McGivering in Baghdad says that six months after the city fell to US-led coalition troops, marked by the symbolic toppling of the giant bronze statue of Saddam Hussein, the mood amongst many people has changed from celebration to frustration.
While Baghdad residents appreciate new basic freedoms and rights, the wave of crime which hit the city after Saddam Hussein has left many, especially women, frightened to leave their homes, she says.
High unemployment, plus dramatic price rises, have added to the sense of frustration and disappointment.
"In every country any citizen wants three things. Food, security and freedom. We have the freedom, let's say, but we don't have enough security and enough food," said Dr Mahmoud Osman, a member of Iraq's new governing council.