With grim predictability, the newspapers and media will speculate whether such an attack could ever take place again. The coverage will be the usual heart-breaking mix of survivors' stories, the ill health of first responders and the cold economics of the cost of the New York memorials. But this year feels different.
America stands on the cusp of a struggle that will dominate its military for the next three years. A struggle that will inevitably come at the cost of American lives lost to terrorist attacks. Two have died already. James Foley and Steven Sotloff met their ends with dignity and incredible bravery. Their deaths remain a terrible waste of life.
But if we can discern a kernel of hope from such evil, it is that their executions prompted action against an organisation wreaking chaos and brutality on an unimaginable scale across the Levant.
On Tuesday 9 September, President Obama met with the bipartisan congressional leadership to discuss his long-awaited strategy to tackle Isis (known as Islamic State). On Wednesday 10 September, the eve of the anniversary of the 9/11 attacks, he addresses the American people and makes the case for another war in the Middle East.
The backroom briefers amid the Beltway have done their jobs admirably. We know from their winks and their nods what the president will say. Obama will say militant extremists from IS will become a direct threat to the United States if they are allowed to control more territory, amass more financial resources and recruit more foreign nationals.
He'll talk about a multilateral force conducting air strikes and the formation of a new Iraqi government that will presumably willingly acquiesce to the Pentagon. John Kerry told reporters the "struggle" against IS could last three years.
And on Sunday 7 September, Obama told NBC: "What I want people to understand... is that over the course of months, we are going to be able to not just blunt the momentum of ISIL [an alternative name for Islamic State], we are going to systematically degrade their capabilities. We're going to shrink the territory that they control. And ultimately we're going to defeat them.''
Which means a sustained campaign of drone, cruise and air force strikes, supported by surgical ground operations from Special Forces. There have already been over 130 missile strikes on IS targets. That number will rise exponentially.
Militarily, militant organisations such as IS and al-Shabaab are the David to the US' Goliath. And the longer the US engage such organisations, the better they get at targeting them.
Just last week, the leader of al-Shabaab, Ahmed Abdi Godane, was killed in an air strike. With the inability to strike back conventionally, such organisations will pour resources into pulling off successful terror attacks.
But the enduring legacy of 9/11 is increased vigilance from the security services, better resources and a determination not to be caught out again.
So will we ever witness an attack on the scale of 9/11 again? The chances are extraordinarily low. The sophistication, funding and planning that went into the attacks were beyond anything IS or al-Shaabab are capable of.
Aviation security, though occasionally probed and less often breached, is secure. The secrets unveiled by WikiLeaks have demonstrated that the ability of intelligence agencies to track communications across the globe is unparalleled.
Governments have improved their recruitment and retention of operatives from diverse ethnic communities. And there is a feeling that a proportion of Western extremists who have travelled to Syria to join IS may regret that decision; after years of the security and relative comfort of living in America or France or the UK, the initial excitement of joining "jihad" recedes. After all, Syria is not a particularly pleasant place to be at the moment.
There are reports of British jihadis quietly testing the waters with the authorities to see if they could turn informant against their "brothers" in return for safe passage back to the UK. Unhappy militants desperate to return to civilisation make excellent human intelligence sources, again reducing the risk of successful large scale terror attacks.
But, despite the increased capability of the security services, governments remain deeply concerned about terrorism. In August, Prime Minister David Cameron raised the UK's terrorist threat level to "severe".
The president, secretary of state and national security adviser have all spoken publicly of an increased threat of a terror attack on American soil. So while the chances of another attack on the scale of 9/11 remains low, it is likely we will witness acts of terrorism in America and the UK committed by militant extremists.
Intelligence officers are terrified of one scenario in particular: a Mumbai style attack in which an individual or group of "home-grown" extremists acquire firearms, or even just use homemade weapons, and rampage through the streets of a city or town without any warning.
If such an individual or group were unknown to the intelligence community – they were not attending a blacklisted place of worship, they avoided internet communities and forums, for example – it would be very difficult to prevent such an attack. Shopping centres are of particular concern, to the extent that the UK has deployed undercover units of SAS soldiers to patrol them.
The Boston bombing, Fort Hood massacre and murder of Lee Rigby are grim examples of the present and future of terrorism in the US and the UK. It will be committed by a country's own citizens who have become disenfranchised and bitter towards their society.
The self-declared imams of militant Islam have attracted the weak willed, the naive, the desperate, the immoral and the mentally ill to their cause. Such individuals have been twisted and manipulated by an ugly, bastardised perversion of a great religion. They have abandoned reason and civilisation for brutality and chaos.
Until the US, along with its allies, confronts and cripples IS, the risk of such organisations orchestrating terror attacks, however brutally simple they are in design and execution, remains high.
But destroying infrastructure and launching endless drone strikes can only be half the battle. Until governments effectively fight the malevolent ideology itself that underpins such organisations, there will always be those willing to commit murder in its name.