While 9/11 failed to bring America to its knees as Al-Qaeda hoped, it ushered in an era of instability, especially in the Middle East, that Islamic extremists have skilfully exploited, analysts say.
By reacting with a doctrine of "Shock and Awe" and invading Iraq in 2003, the US committed a series of missteps that indirectly helped foment jihadism, say critics.
Founded in the 1980s, Al-Qaeda was gravely weakened after being expelled from Afghanistan in 2001 by the US-led invasion of the country that had harboured the terror group's leader Osama bin Laden.
But the occupation of Iraq offered them a new opportunity, underlining how jihadist groups have resisted attempts to vanquish them and have now expanded into franchises in the Middle East, Asia, the West and parts of Africa.
"September 11 was the culmination of several years of planning by Al-Qaeda of 'the big one'," Didier Le Bret, who recently resigned as France's national intelligence coordinator to run in next year's parliamentary election, told AFP.
"Above all it marked the start of a realisation (by the Americans) of their vulnerability on home soil. And that is something they cannot bear."
He said the invasion illustrated the dangers of acting first and worrying about the consequences afterwards.
Among the consequences was the damage done to America's reputation by the torture of prisoners at Abu Ghraib prison in Iraq and Guantanamo Bay, the camp opened by George W Bush at a US base in Cuba to hold "enemy combattants" captured around the world.
For Le Bret, the conflict in Iraq was "an unfinished war, built on a lie" that ended up destabilising the whole region.
Jean-Pierre Filiu, professor of Middle East studies at Sciences-Po University in Paris, said it also squandered support for the US among its allies.
"The US enjoyed unprecedented international solidarity in its campaign against the Taliban and Al-Qaeda (in Afghanistan)," he said. "But after that campaign, which was crowned with success within a few weeks, the Neoconversatives imposed a strategy of a 'global war on terror' which reignited global jihad."
Tai chi terror tactics
The US presence in Iraq made it a magnet for jihadists, with Al-Qaeda in Iraq becoming a major force in the post-war insurgency that later morphed into the Islamic State (IS) group.
IS extremists, seeing power vacuums created through weak governance in Iraq and a civil war in Syria, grabbed territory in both countries after declaring their "caliphate" for Muslims in 2014.
Spurred by social media and the forces of globalisation, they have since encouraged Muslims to take up arms against the West which they see as an ideological enemy.
While conducting a campaign of mass murder and terror in territories under their control, they have spread their tentacles worldwide, inspiring or carrying out attacks in France, Germany, the United States, Turkey or Bangladesh.
"Daesh, or the so-called Islamic State, was born out of the alliance of two totalitarianisms, that of Al-Qaeda and the ousted regime of Saddam Hussein," Filiu said.
"Instead of taking the measure of this new threat, Barack Obama was for a long time in denial, allowing the emergence of a 'terror caliphate' that has spread around the world," he argued.
Obama's reticence to commit the US to a new battle in the Middle East reflected the caution of a nation scarred by Iraq, where thousands of US troops and tens of thousands of Iraqi civilians died during the post-invasion insurgency.
Bin Laden's son said 9/11 was aimed at setting up a grand showdown with America in Afghanistan where the Soviet army had been defeated by US-backed mujahideen.
"I was surprised the Americans took the bait", Omar bin Laden, one of 11 sons, told Rolling Stone magazine in 2010, a year before his father's death at the hands of US special forces.
By forcing Washington out of its isolationism and luring it into Iraq, bin Laden was using what historian Yuval Noah Harari calls the "tai-chi masters method."
"The terrorists hope that even though they can barely dent the enemy's material, power, fear and confusion will cause the enemy to misuse its strength," the author of Sapiens, a history of humanity, wrote in The Guardian newspaper last year.
"Terrorists calculate that when the enraged enemy uses its massive power against them, it will raise a much more violent military and political storm than the terrorists themselves could ever create."