Some events, such as the assassination of JFK and the death of Princess Diana, stay seared into our memory. No one alive at the time will ever forget where they were when they heard the news.
Others, such as the assassination of Archduke Ferdinand and his wife Sophie in Sarajevo on June 28, 1914, change the world irrevocably in an instant of time. By the time the smoke had cleared countless millions had died, the monarchs of Germany, Austria and Russia had been deposed, and national borders had been redrawn on the largest scale ever known.
The attacks on the World Trade Center, the Pentagon and the US Capitol building 20 years ago on September 11 did both.
Once the reality of the incredible scenes on television had sunk in, billions of people around the globe knew nothing would ever be the same again. America had been attacked, almost 3000 people - the vast majority of whom were innocents just going about their daily lives - had been killed, and the nation was plunged into mourning.
Millions had been traumatised by scenes of almost unprecedented horror. Many survivors and witnesses are still held in thrall by PTSD as a result of that awful day.
While the globe held its breath to see how Washington would respond, a small minority in Congress unsuccessfully argued for the calamity to be used to reconnect with the Islamic world given the outpouring of support for the US after the towers fell.
Instead George W. Bush missed the moment, through his indifference to civilian casualties during the invasion of Afghanistan and then leaving that job unfinished while going to war with Iraq.
The bungled Western departure from Kabul in the wake of the unexpectedly rapid Taliban victory was the inevitable outcome of often ambiguous, conflicting and ill-considered US policies in both countries.
America is not the only nation that has lost face through the abandonment of friends. The credibility of the other Western allies, including Australia, has taken a big hit as well.
If the intent of the invasion was to stop Afghanistan from being used by jihadists for attacks on the West, it appears to have failed given the rise of ISIS-K. There seem to be as many terrorists as ever.
If it was to create a new and democratic nation where there had never been one before, and in which the rights of women and minorities were respected, the new order didn't even come close. The US-installed puppet government was riddled by corruption and malfeasance on an epic scale.
When the Taliban came knocking, the rats at the top abandoned their sinking ship of state and the long unpaid troops and police deserted in droves.
With aid agencies and other humanitarian organisations forbidden from dealing with the Taliban, and the country's offshore monetary reserves now frozen, Afghanistan seems destined to experience a ghastly humanitarian catastrophe.
Ordinary Afghans are the ones who are paying the price for September 11, not the Taliban who sheltered al-Qaeda and facilitated the carnage. We can only speculate on what might have happened if the West had used the trillions squandered on the post-September-11 wars to counter the ignorance, poverty, inequality and disadvantage in the Muslim world that makes it so easy for jihadists to recruit fresh blood.
Who knows? If the US had invested in genuine nation-building in Afghanistan after its initial rout of the Taliban in 2001 and 2002, it is possible something more positive could have come as a consequence of the almost 3000 people killed on September 11 - and hundreds of thousands of other lives may have been saved.
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