US President George W Bush has said that the sacrifices being made to fight insurgents in Iraq are vital to the future security of the United States.
Speaking at an air force base in North Carolina, he urged Americans to stand firm on "the latest battlefield in the war against terrorism".
He said the only strategy to tackle militants was to "defeat them abroad before they attack us at home".
He had no plans to send more troops to Iraq or schedule withdrawal, he said.
He said setting a timetable would simply enable insurgents to wait for a US departure.
"We will stay in Iraq as long as we are needed - and not a day longer," Mr Bush said.
The president said he would send more troops if military commanders said it was necessary.
But he added that increasing US troop numbers "would undermine our strategy of encouraging Iraqis to take the lead in this fight".
Mr Bush's address came on the first anniversary of the handover of power to an Iraqi interim government.
He attempted to answer growing criticism of the US presence, as opinion polls indicated disapproval of his handling of the war in Iraq.
Mr Bush's approval ratings have fallen to their lowest point in his presidency.
"Is the sacrifice worth it? It is worth it and it is vital to the security of our country," he said.
"The American people do not falter under threat - and we will not allow our future to be determined by car bombers and assassins.
"The only way our enemies can succeed is if we forget the lessons of September 11."
More than 1,000 people - mostly Iraqis - have been killed since the new Iraqi government was formed in April.
The US death toll has pushed past 1,700 since the conflict began in March 2003.
Senate Democratic leader Harry Reid said Mr Bush's emphasis was misplaced and his "numerous references to September 11 did not provide a way forward in Iraq".
He said the president's speech reminded Americans that "our most dangerous enemy, namely Osama Bin Laden, is still on the loose".
US statements on Iraq have given mixed messages over the past weeks.
Vice-President Dick Cheney has said the insurgency is in its "last throes", while Defence Secretary Donald Rumsfeld has warned it could last up to 12 years.
The Bush administration strategy is based on support for the political process in Iraq.
But the BBC's John Leyne in Baghdad says that while a majority of Iraqis reluctantly want the US presence to continue, a substantial minority want to get the troops out.
Meanwhile the Iraqi security forces are still years away from being able to tackle the insurgency on their own, he says.