September 11th 2001 was also, famously, "the day the world changed". What is harder to assess, even almost a decade on is what changed because of 9/11 - in politics, economics and security - and whether some of what we attribute to it might have happened anyway.
It was not inevitable that the attacks would succeed: specifically, the plotters could have been arrested, perhaps while taking their flying lessons in the months before the attack. More generally, the complicated choreography of a terrorist "spectacular" required a great many things to go right for the perpetrators. That is one of the rules of 'assymetric warfare' - that the terrorists only need to get lucky once. But Al-Qaeda have had no similar success in the United States in the eight years since, although there have been lethal Islamist terror attacks in Madrid and London.
To be clear, a world without 9/11 would not have been one in which there was no terrorist threat, but it might have been one in which the central issues of international politics were different. Al-Qaeda was, of course, active long before 9/11. But even large-scale attacks such as the African Embassy bombings of 1998 did not have anything like the same geopolitical impact as the attacks of September 2001. Bin Laden was the 'most wanted' among specialist intelligence agencies; he was not among the half a dozen most notorious figures in global politics.
So what did 9/11 change in international politics? It is often difficult to judge. Here are some areas where a case that things might have been very different end up without 9/11.
1. US politics: Would George Bush have been re-elected?
The unpopularity of George W Bush by the end of his second term in 2008 means that it is sometimes forgotten how the impact of 9/11 was, despite question marks over the immediate administration response, to considerably strengthen the political position of the President.
The September 2001 crisis significantly reduced, and removed from the centre of US political debate, the legitimacy question over the Bush Presidency of the contested election of 2000, as the US rallied around the President and Commander-in-Chief. If that had not happened, might a Bush-Gore rematch have been more likely than a Bush-Kerry election in 2004?
Without 9/11, and the War Presidency campaign of 2004, would George W Bush have won a second term - or might that very close election have seen the Republicans lose?
2. Would the Iraq war have happened?
Given the lack of any credible linkage between Al Qaeda and Saddam Hussein, there is a good case for arguing that Washington hawks like Cheney and Wolfowitz would have been convinced that the on-going US-Iraq stand-off (with miliitary action in 1998, and the on-going sanctions regime) needed to end in "regime change".
But there can be little doubt that the changed climate of US opinion made it easier to make a public case in the United States for a military confrontation with Iraq. And this did have majority support in Congress and among the US in 2003, with significant controversy but nothing like the level of opposition that there was outside the US?
So without 9/11, would the Bush administration have won sufficient domestic political support for an Iraq war in 2003?
3. British politics: Might Blair have held a Euro referendum?
Would British politics have been different? Again, perhaps much depends on the question about Iraq.
Iain Duncan Smith, elected Tory leader on September 11th 2001, saw domestic politics disappear for a couple of years. But it seems unlikely that this prevented him fashioning a Tory revival.
Tony Blair's agenda shifted considerably, firstly, with his liberal intervention agenda becoming his central public argument, and then Iraq dominating everything. What may be little remembered now is that, at the TUC conference on September 11th 2001, he was about to give a major speech on Europe. He may have thought this would be the defining issue of his second term.
More broadly, there has undoubtedly been a major impact on public discussion of identity, citizenship and faith - arguments that have often become sharply and dangerously polarised.
4. Transatlantic relationships: would there have been a rift with Bush?
It might well be argued that 9/11 did not itself causes significant transatlantic rift - with close cooperation in the first six months over a multilateral response focused on Afghanistan and a broader anti-terrorist strategy - and that divisions over the Bush administration's approach to the "war on terror" were much deeper once the focus shifted to Iraq after the fall of the Taliban.
However, if the 9/11 plot had failed, there would have been continued and on-going attempts to organise some sort of 'spectacular' in major western cities. The first major Islamist terrorist attack on a western city might have been in Europe rather than in North America, for example the successful attacks on Madrid or London, or other failed efforts in Germany and France. Would that have changed the nature of the international response to terrorism?
5. Afghanistan: would the Taliban have retained power after 2001?
It was on September 9th 2001 that anti-Taliban leader Ahmed Shah Masoud was assassinated in Afghanistan. The international response to 9/11 led directly to the fall of the Taliban/Al-Qaida regime through a UN endorsed international operation. While there were long-standing UN resolutions, without 9/11, how likely is it that there would have been any direct western intervention in Afghanistan. What would the impact have been on Afghanistan's internal power struggles without that - and how might this have reshaped the broader politics of the region, including the role of Pakistan, and the global impact of Afghanistan's internal strife?
6. Would Islamist militancy be weaker?
The Guardian today reports that Al Qaeda have been weakened organisationally over the last eight years.
But 9/11 perhaps also changed a great deal the nature of what "al Qaeda" is - an argument consistently made by Jason Burke, perhaps the western reporter who has followed radical Islamist militancy most closely over the last twenty years:
we need to face up to the simple truth that Bin Laden, al-Zawahiri et al do not need to organise attacks directly. They merely need to wait for the message they have spread around the world to inspire others. Al-Qaida is now an idea, not an organisation.
If bin Laden lost his training camps, he gained enormously in global profile and notoriety: he might well consider that the idea of a "clash of civilisations" between the west and the Muslim world has gained ground in the west too. Without 9/11, would the nature of Islamist radicalism have changed less? Would its appeal have been less?
7. In the long-run .... did 9/11 change the world less than we tend to think it did?
There is a quite plausible, sceptical case that 9/11 changed rather less than is often claimed. The economic boom (and bust), the environmental crisis, many geopolitical questions (the rise of China, Israel and Palestine, Iran) and much of our domestic political arguments would have remained, though perhaps even only tangentially related events may often have been altered in unpredictable ways.
It seems unlikely that an event which dominated politics, public debate, media discussion and the attention of governments so much did not have a very significant impact in many ways.The truth is perhaps that we can never know how, or quite be able to join the dots, still less work out where we would end up.
Iraq ended up defining the political decade in the west more than 9/11 itself. So how much the world changed because of 9/11 depends on whether the course of events which led to the Iraq war night have been significantly different.
If no 9/11 did mean less chance of a Bush second term, it would mean no Obama Presidency either.
Ultimately, the events of parallel universes are never available to us. But asking "what if" might help us to reflect on the choices that were made too, and those we face in future.