US President George W Bush has urged Americans to unite behind the "war on terror" in a speech marking five years since the 11 September attacks.
Mr Bush cast the "war on terror" as an historic battle of ideas
The president talked of "a struggle for civilisation" and said the safety of the nation depended on the outcome of "the battle in the streets of Baghdad".
The Middle East, he said, faced "terrorist states and radical dictators armed with nuclear weapons".
US Democrats accused Mr Bush of playing politics ahead of November elections.
One leading Democrat, Senator Edward Kennedy, said Mr Bush should be ashamed of using a national day of mourning for political gain.
In a prime-time television address from the Oval Office, President Bush said the "war on terror" was much more than a military conflict.
"It is the decisive ideological struggle of the 21st Century and the calling of our generation," he said.
"It is a struggle for civilisation. We are fighting to maintain a way of life enjoyed by free nations."
Eye on election
Mr Bush was speaking hours after attending solemn ceremonies at sites in New York, Washington and Shanksville, Pennsylvania, where Islamist suicide attackers crashed hijacked airliners in 2001, killing nearly 3,000 people.
His speech was dignified in tone and bearing, but the message was intensely partisan - a fact that will not be lost on his audience, reports the BBC's Justin Webb in Washington.
"We must put aside our differences and work together to meet the test that history has given us," Mr Bush said.
He praised the heroes of 11 September, while for Osama Bin Laden, al-Qaeda's fugitive leader, there was a pledge to bring him "and other terrorists... to justice".
However, as our correspondent notes, the speech also had the aim of shoring up Mr Bush's Republican party before the mid-term Congressional elections.
To help with that, he needed to convince a largely sceptical nation that the war in Iraq was part of the war on al-Qaeda.
"I'm often asked why we are in Iraq when Saddam Hussein was not responsible for the 9/11 attacks. The regime of Saddam Hussein was a clear threat... and after 9/11 Saddam's regime posed a risk that the world could not afford to take," Mr Bush said.
"The world is safer because Saddam Hussein is no longer in power."
Mr Kennedy accused the US leader of using the anniversary to bolster support for an unpopular war.
"The president should be ashamed of using a national day of mourning ... to seek support for a war in Iraq that he has admitted had 'nothing' to do with 9/11," he said.
Al-Qaeda, the militant network held responsible for the 11 September attacks, has issued a new threat of attacks which it said would target Gulf states and Israel in particular.
In a video released on the eve of the anniversary, Ayman al-Zawahiri, al-Qaeda's second-in-command, said Western forces were doomed to defeat in Iraq and Afghanistan.