It was, by any measure, a stunning blow for the world's only superpower - the United States.
On September 11, 2001, the most militarily capable nation in the world was powerless to prevent attacks on its soil against the very symbols of its power and prestige by a transnational terrorist group, al Qaeda.
For the Bush administration, 9/11 was a transformative event that changed everything. The world on September 12 was deemed to be a very different place from what it had been on September 10.
But this interpretation of 9/11 was profoundly mistaken, and the policy consequences proved to be disastrous.
The global security environment had in fact been radically changing since the end of the Cold War.
In many ways, the controversial US-UN humanitarian intervention in Somalia in the early part of the post-Cold War era was a defining moment for Washington and the beginning of a road that led to 9/11.
The bloody battle in Mogadishu in October 1993, which took the lives of 18 US servicemen, taught the Clinton administration that failed or failing states were not vital to the national security interests of America.
But al Qaeda's involvement in the episode known as Black Hawk Down convinced the bin Laden network that "the Americans will leave if they are attacked".
Between 1993 and 2000, American personnel and interests were the target of violent terrorist attacks from al Qaeda or its associates in places such as New York, Addis Ababa, Riyadh, Khobar, Nairobi, Dar es Salaam and Aden.
So the 9/11 attacks did not come out of a clear blue sky, but they did provide an opportunity for policymakers in Washington to finally come to terms with a globalised security environment that generated threats from transnational terrorist groups like al Qaeda.
However, the Bush administration largely failed to take the chance to realign US national security policy.
By claiming that 9/11 had suddenly changed the world, the Bush administration conveniently obscured the origins of the attacks and felt free to declare an all-out war on what was called global terrorism.
The assumption that it was possible to wage war with terrorism led to an almost exclusive military focus by the Bush administration in the conflict with al Qaeda.
Beginning with President George Bush's State of the Union address in January 2002, in which Iraq, Iran and North Korea were labelled the "axis of evil", the Bush administration emphasised the ideas of US global primacy and pre-emptive war.
In March 2003, the Bush administration bypassed the UN Security Council and launched a US-led invasion of Iraq, a country that had nothing to do with the 9/11 atrocity.
Far from weakening global terrorism, the US invasion spawned a major insurgency in Iraq; provided a foothold for al Qaeda operatives in the country; fuelled anti-American sentiments in the Islamic world; and, by 2006, directly contributed to a resurgence of Taliban and al Qaeda forces in the Afghanistan area.
In Iraq and elsewhere, the Bush team after 9/11 had few qualms about privileging America's national security interests over concerns such as human rights and the rule of law.
Yet these principles lie at the heart of the liberal democratic system and play a key role in challenging terrorist groups like al Qaeda that are dedicated to destroying such norms.
Allegations concerning US violation of human rights in Afghanistan, the almost indefinite detention of terrorist suspects at Guantanamo Bay and shocking reports of abuse at the US-run Abu Ghraib prison in Iraq provided propaganda windfalls for al Qaeda and seriously damaged America's international image.
But if Mr Bush was still reluctant after 9/11 to recognise the limitations of American power, the Obama Administration felt it had little choice but to face this reality.
America alone, President Barack Obama argued, could not meet transnational security threats from al Qaeda or elsewhere, and he embarked upon a process of relationship rebuilding in the international arena to confront these dangers.
In specific terms, the Obama Administration jettisoned the "war on terror" rhetoric; withdrew all US combat troops from Iraq; pursued a more even-handed stance on the Israeli-Palestinian conflict; escalated the ideological battle against Islamic terrorism; and intensified the war against al Qaeda and the Taliban in their strongholds of Afghanistan and Pakistan.
While some strands of Mr Obama's national security policy have not lived up to expectations, the overall impact has been to squeeze al Qaeda to its lowest level of operational capability since the late 1990s.
First, on the ideological battlefront, Mr Obama has proved a much tougher opponent for al Qaeda than Mr Bush. Within 18 months of coming to office, Mr Obama made landmark speeches in Istanbul and Cairo in which he made it plain to enthusiastic audiences that it was al Qaeda, not the US, that was the enemy of mainstream Islam.
For Mr Obama and his team history offers a clear verdict for the likes of al Qaeda. In an interconnected world, it is the ideas of democracy, not dictatorship and political fundamentalism, which have mass support. The recent uprisings in Tunisia, Egypt, Syria and Libya seemed to confirm that al Qaeda had been sidelined in the Middle East.
Second, on the military battlefront, the Taliban and al Qaeda have found themselves under greater and more sustained pressure in Afghanistan and Pakistan than at any time since late 2001.
In 2009, the deployment of about 50,000 new US troops in Taliban-dominated areas like Kandahar and Helmand facilitated a much more aggressive counterinsurgency campaign in Afghanistan.
The Taliban experienced displacement, and high levels of casualties and desertions.
While observers worried that Mr Obama's Afghan approach had ensnared America in another Vietnam, the Administration indicated the US-Nato operation would end in 2014 and that it was seeking the bargaining power to negotiate that would come from a demonstrated capacity to hurt the Taliban.
Meanwhile, the Obama Administration dramatically escalated the war against the al Qaeda leadership in the region, especially through the use of drone attacks in Pakistan's tribal lands.
During the last 20 months, more than a third of al Qaeda's top leadership, including Osama bin Laden, have been killed or detained.
Since 2009, the Obama leadership has realigned US national security policy to more effectively counter the terrorist threat from al Qaeda.
But the process of adjusting policy is incomplete and the threat from transnational terrorism remains.
In the Middle East, Mr Obama's efforts to advance the establishment of a Palestinian state - a move considered vital to stem the hatred that fuels Islamist terrorism - have faltered after Israeli Prime Minister Benjamin Netanyahu expressed reservations about the idea in the US.
It will take leadership skill and political courage for the Obama Administration to overcome such constraints and ensure that America lives in a more secure world.
• Robert G. Patman is a professor of international relations at the University of Otago
