Five times, President Barack Obama has come before the American people to reckon with the legacy of the 9/11 terrorist attacks on the anniversary of that awful day. Each time, he has tailored his message to the moment, at different points stressing themes of service, resilience, tolerance, reconciliation.

This year, with the threat of the Islamic State militant group looming, Obama's rhetoric of remembrance is circling back to earlier days and striking a less triumphant tone.

In an address to the nation about the terrorist threat, a day ahead of Thursday's 9/11 anniversary, Obama said, “We cannot erase every trace of evil from the world, and small groups of killers have the capacity to do great harm. That was the case before 9/11, and that remains true today". He was speaking to a public that is increasingly worried about Islamic extremism.

Six in 10 Americans now are very concerned about the rise of extremism around the world, the largest share since 2007, according to a new survey from the Pew Research Center “The picture of the United States winding down the war on terror has become harder to sell in the last couple of years,” says Peter Feaver, a Duke University expert on war and public opinion.

“The old rhetoric that we are just about to win the war on terror can't be sustained."

In his first turn at the presidential bullhorn on 9/11, Obama tried to buck up war-weary Americans with a call to service and common purpose. “Let us renew the true spirit of that day _ not the human capacity for evil, but the human capacity for good,” Obama said at a Pentagon memorial service.

A look back at how Obama has framed his 9/11 message over the past five years.

2009: A new president stresses service, resolve

Obama called on Americans to “summon once more that ordinary goodness of America, to serve our communities, to strengthen our country to better our world". Obama said he was “mindful that the work of protecting America is never finished".

2010: A call for tolerance amid growing religious tensions

A Florida preacher's call to burn Qurans and growing tension over plans to build a mosque near ground zero in New York hung over the ninth anniversary of 9/11. Obama responded with a call for tolerance.

“They may seek to spark conflict between different faiths, but as Americans we are not _ and never will be _ at war with Islam,” Obama said. “It was not a religion that attacked us that September day; it was al-Qaida, a sorry band of men which perverts religion. And just as we condemn intolerance and extremism abroad, so will we stay true to our traditions here at home as a diverse and tolerant nation."

2011: Taking stock of progress on 10th anniversary of attacks

Obama used the 10th anniversary of the 9/11 attacks to take the measure of a decade's progress.

“These past 10 years have shown that America does not give in to fear,” he said. “Our people still work in skyscrapers,” Obama said.

“Our stadiums are still filled with fans, and our parks full of children playing ball. ... This land pulses with the optimism of those who set out for distant shores, and the courage of those who died for human freedom. " Overall, he said, “10 years later, I'd say America came through this thing in a way that was consistent with our character. With the wars in Afghanistan and Iraq drawing to a close, he looked forward to 'a future of peace'. "“

2012: Taking a break from politics in an election year; tragedy strikes in Benghazi

Obama and GOP rival Mitt Romney took a break from presidential politics on 9/11, with Obama and his wife walking among the graves of the most recent war dead at Arlington National Cemetery.

“It's because of their sacrifice that we've come together and dealt a crippling blow to the organisation that brought evil to our shores,” Obama said, delivering an upbeat assessment of the waning terror threat. “Al-Qaeda's leadership has been devastated and Osama bin Laden will never threaten us again. Our country is safer and our people are resilient. — Within hours, word surfaced that four Americans had been killed in an attack on a US compound in Benghazi, Libya.

2013: Shadows of Syria hang over anniversary

The prospect of more US military action in the Middle East hung in the air. “Let us have the strength to face the threats that endure, different though they may be from 12 years ago, so that as long as there are those who would strike our citizens, we will stand vigilant and defend our nation,” Obama said.

He spoke on the morning after an address to the nation in which he defended a possible military strike on Syria in retaliation for a deadly chemical weapons attack. The airstrikes never happened, and Syria began dismantling its chemical weapons stockpiles.

But there were new allegations of chlorine attacks in Syria over the summer. '2014: The threat of airstrikes against Syria is back as Obama develops his strategy to counter militants of the Islamic State group.'