In September 2001, members of al-Qaeda managed, largely because of luck, to pull off a risky terrorist act that became by far the most destructive in history.
The peculiar trauma evoked by the attack has proven to be lasting: a recent survey revealed that more than 70 per cent of Americans believe a terrorist attack inflicting large numbers of deaths in the United States is very or somewhat likely in the near future — the same percentage as in the days following 9/11.
This has impelled a US reaction that has been substantially delusional, one massively disproportionate to the threat that al-Qaeda has actually presented, either as an international menace or an inspiration to home-grown amateurs.
As anthropologist Scott Atran has put it: "Perhaps never in the history of human conflict have so few people with so few actual means and capabilities frightened so many."
The chance that an American will perish at the hands of a terrorist at present rates is 1 in 3.5 million per year. For Australians, it is one in 7.1 million.
There is less than one chance in 20 million that a plane flight will be hijacked or attacked by terrorists, and one chance in 90 million that a passenger will be killed by terrorists.
For the terrorism risk to border on becoming "unacceptable" by established risk conventions — that is, to reach an annual fatality rate of 1 in 100,000 — the number of fatalities from all forms of terrorism in the United States would have to increase 35-fold, and in Australia more than 70-fold.
Discussions about terrorism rarely mention such statistics.
Nor do they consider how much safer do we want to be, and at what cost? Such considerations are crucial to any rational discussion on counter-terrorism, and more so in the present age of austerity.
Yet a year into the second decade after 9/11, those questions are not being systematically considered.
The low frequency and generally limited severity of terrorist attacks makes the benefits of the huge enhanced counter-terrorism expenditures since 9/11 difficult to justify by any rational and accepted standard of cost-benefit analysis.
US domestic security since 2001 has increased by over $US1 trillion, and analysis suggests these enhanced expenditures would have had to prevent one very substantial attack every day to be cost-effective.
Like vandalism and crime, terrorism can be carried out by an individual or small group and can therefore never vanish from the human experience: a condition of zero vulnerability is impossible to achieve.
However, some specific counter-terrorism measures do appear to be cost-effective.
The hardening of cockpit doors and the installation of secondary barriers on airliners seem to be so, although the provision for air marshals is not.
We find that many protective measures are a waste of money in part because there is a displacement effect, a transfer of risk: terrorists can choose, and change, their targets, depending on local and immediate circumstances.
Thus, protective measures that involve static defence are likely to be easily circumvented or softer targets found — of which there are many.
On the other hand, at least some of the investments in intelligence and policing may pay off because these resources are flexible and can respond quickly to emerging threats, and because disrupting attacks during the planning stage is often less costly.
Rationally reassessing homeland security expenditures — and even, perhaps, reducing them — would seem to be possible politically.
The United Kingdom, which seems to face an internal threat from terrorism that is considerably greater than that for the United States, appears nonetheless to spend proportionately much less than half as much on homeland security, and the same holds for Canada and Australia.
Yet politicians and bureaucrats there do not seem to suffer threats to their positions or other political problems because of it.
However, those in charge do not seem to be very interested in trying, even 11 years after 9/11, to carry out a systematic reassessment.
In all, the monster of terrorism and the internalised fears it has inspired may prove to be eternal. More than 60 per cent of Australians feel the war on terror will never end and they may be right.
If so, the additional $US1 trillion expended to deal with terrorism in the US in the first decade after 9/11 might well prove to be simply a down payment.