The attacks on the World Trade Centre in New York and on the Pentagon in Washington 20 years ago still affect Australia.
They weren't just a shocking one-off event in a far away place. They changed the way Australia operates. They changed our lives.
The attacks got us into two wars, one of which has only just ended in what looks much like defeat.
They spawned "The War on Terror" which generated a relentless ratcheting up of government surveillance of its own citizens, according to Lydia Khalil of the Lowy Institute.
"In Australia, we've passed a huge amount of surveillance and counterterrorism legislation.
"Advances in technology coupled with a pervasive ongoing sense of anxiety stemming from the September 11 attacks have led citizens to, by and large, accept these measures and normalise mass surveillance."
The attacks were organised by Al Qaeda whose leaders, including Saudi-born Osama bin laden, were based in Afghanistan, protected there by the Taliban which has just retaken power.
It's not surprising that the US and its allies including Australia went after the perpetrators in Afghanistan, but it is still far from clear why Iraq was attacked. The regime of Saddam Hussein in Baghdad was no doubt drenched in blood but intelligence agencies have not linked it to the Al Qaeda hijackers.
Lydia Khalil thinks that part of the current erosion of democracy around the world stems from the post-9/11 wars. Methods sometimes used by the allies were brutal, sometimes illegal and far from democratic ideals.
"Internationally, you had interventions and counterterrorism measures carried out by democratically-elected governments that included torture, rendition and military occupation in the name of security and securing freedoms," she said.
"And domestically, civil liberties were violated in the name of counterterrorism and security."
Apart from the big picture, our daily lives remain affected: go to an airport and see the security machinery. Travellers are now used to being herded, shoeless, their bodies patted down.
The reputation of Australian forces was changed by the aftermath of the wars the attacks sparked.
Before Afghanistan and Iraq, the futile heroism of the diggers at Gallipoli was the over-riding image of the Australian military. Today, the image is clouded by allegations of war crimes in Afghanistan.
And the human suffering of troops and civilians needs to be counted.
In the attacks themselves, it is reckoned that 2,996 people, including 10 Australians, died - 2,606 in the World Trade Center and 125 in the Pentagon. There were 265 on the four planes, including the 19 hijackers who committed simultaneous murder and suicide.
The fourth plane, probably heading for the Capitol in Washington, crashed when passengers tried to overcome the hijackers.
There was unified global outrage. Prime Minister John Howard who was in Washington on the day of the attacks announced that Australian troops would stand shoulder-to-shoulder with American troops and their comrades from 51 countries.
Twenty years on, the Taliban which sheltered Al Qaeda are back in power in Kabul. Osama bin Laden is dead and buried at sea but terrorism is not defeated.
But it has been bruised and knocked back, according to Dennis Richardson, the Australian ambassador to the United States from 2005 to 2010 and before that Director-General of the Australian Security Intelligence Organisation.
In recent years, attacks by Islamists have tended to be lone affairs and not the work of organised groups.
War was inevitable, he thinks. "The US was attacked in its own homeland and the then-Taliban refused to give up those responsible for the attack so what then happened was inevitable."
He doesn't like the word "defeat".
Despite the harrowing pictures of those left behind in the brutal hands of the Taliban, Mr Richardson thinks there were gains from the war: it would be unlikely for the current Taliban to shelter terrorists in the same way as its predecessors did.
A lesson has been learnt, he hopes. "Do you think the Taliban second time round would let a group like Al Qaeda set up in Afghanistan?"
And the people of Afghanistan, particularly women, benefited from the presence of the coalition over the previous 20 years.
In Dennis Richardson's view, September 11 transformed global politics by prompting the US to switch its attention to the Middle East.
"Australian counter-terrorism moved from the periphery of government policy to the centre of government policy," the former head of ASIO said.
In November, 2001, Al Qaeda described Australian forces in Muslim-majority Indonesia as a "crusader force".
The Bali bombings happened a year after the 9/11 attacks. Two years after Bali, on September 9, 2004, the Australian embassy in Indonesia was bombed, two days short of the anniversary of the 9/11 attacks.
Foreign policy experts debate whether the focus on terrorism meant the US took its eye off Russia and China.
"China would no doubt have risen in power and stature without the attacks," according to Professor Michael Cox of the London School of Economics.
On his reading, the United States wanted to "build bridges" to countries like China in return for help (or the absence of hindrance) in the War on Terror.
"This may have been a wise and necessary policy back then.
"But while the United States remained bogged down fighting wars across the Middle East, from which it has been trying to extract itself ever since, China steadily progressed along its trajectory towards major power status unimpeded."
He thinks the "serious long-term challenge" of China might prove to be "every bit as difficult to deal with - if not more so - as 9/11."