Ten years after its attacks on the US, Al Qaeda is in deep crisis, reeling from the death of its founder, riven by internal tensions and desperately struggling to remain relevant in a time of momentous political upheaval in the Middle East.
While US and European security officials say that the group's core has disintegrated and that it is almost certainly incapable of mounting another attack on the scale of September 11, its offshoots, particularly in Yemen, have developed as a potent threat to the US and its allies, as well as regional governments they view as pro-western.
Still, the US president, Barack Obama, and other top US officials are confident that Al Qaeda is in decline, thanks largely to a campaign of unmanned drone strikes against the group's strongholds.
Mr Obama said on Saturday that while Al Qaeda would "keep trying to hit us again", the US is better prepared for a terrorist attack than ever before.
He pointed out that since becoming president in 2009 "more senior Al Qaeda leaders have been eliminated than at any time since 9/11". America, he proclaimed, "is stronger and Al Qaeda is on the path to defeat".
Leon Panetta, the US defence secretary and former CIA director, said in July the US was "within reach of strategically defeating" Al Qaeda if it maintained "maximum pressure" on its remaining key leaders, who are estimated to number between 10 to 20 figures in Pakistan, Yemen, Somalia and North Africa.
A month later, one of the chief instruments of Al Qaeda's decline - the unmanned drone - was credited with the killing of Al Qaeda's second-in-command, Atiyah Abd Al Rahman, in Pakistan's tribal areas.
Al Qaeda is "on the ropes", the White House counterterrorism chief, John Brennan, proclaimed after Rahman's death.
Many experts agree Al Qaeda, which was founded in the late 1980s, is weaker than it has ever been.
Jason Burke, author of the recently published, The 9/11 Wars, said: "Bin Laden's dreams of mobilising an international jihad against the US and its authoritarian allies in the Arab world proved unattainable, while his strategy of using spectacular terror attacks was self-defeating."
The deaths of countless Muslim civilians at the hands of Islamist militants from Baghdad to Amman and Islamabad since September 11 cost it support.
"Physically, geographically, politically, socially, culturally and economically, Al Qaeda is marginalised in the Muslim world," Mr Burke said.
Once a centralised and hierarchical organisation, Al Qaeda has mutated into a diffuse network that US officials acknowledge still represents a challenge.
The Al Qaeda affiliates in Yemen and North Africa are seen as the best-organised branch of Osama bin Laden's original network. Al Qaeda's subsidiary in Iraq, though weakened over the years, also remains capable of attacks, as the coordinated bombings that killed 74 people on August 15 showed.
It is the group's Yemen offshoot, however, that appears most capable of launching attacks at home and abroad. There is "no question" that Al Qaeda in the Arabian Peninsula (Aqap) "has risen to the top of the list" as a source of terrorist threats, Mr Panetta said last week.
Aqap's most high-profile figure and key recruiter is Anwar Al Awlaki, a Yemen-based, American-born cleric who is fluent in English and has gained a following with his lurid if eloquent internet sermons.
Mr Awlaki, 40, is said to have masterminded the thwarted Christmas Day bombing of a US airliner over Detroit in 2009 and an attempt last year to blow up two Chicago-bound cargo planes with photocopier toner cartridges packed with explosives.
On the military front, Al Qaeda is facing formidable obstacles.
The US has The CIA's Counterterrorism Centre (CTC), which had 300 employees on the day of the September 11 attacks, now exceeds Al Qaeda's core membership around the world, with some 2,000 staff, the Washington Post reported earlier this month. The intelligence agency's drone programme has killed more than 2,000 militants and civilians since 2001, the newspaper said.
And a Rand Corporation report last month found that Al Qaeda's use of the internet to recruit home-grown militants had largely failed, with only 10 of 32 plots going beyond the discussion stage and six of those broken up by federal law enforcement agents.
Al Qaeda is faring no better in the battle of ideology. To the group's dismay, Islamist groups in Egypt, Tunisia and Libya have been at pains to affirm their commitment to peaceful, multiparty democracy. Three days before Egypt's president Hosni Mubarak was toppled in February, its Iraqi branch urged Egyptians not to replace dictatorship with "filthy secularism", "infidel democracy" or "pagan nationalism".
Adding to Al Qaeda's woes are internal frictions, exacerbated by regional and generational differences. Despite being bin Laden's deputy since 2001, Mr Zawahiri's accession to the top spot was not smooth.
Aqap only belatedly pledged its allegiance to the Egyptian-born medical doctor. Al Qaeda's branches in Iraq and North Africa welcomed Mr Zawahiri's appointment only through spokesmen.
"Al Qaeda seems to be chronically ill at the very least," Mr Burke said. "But there's a danger of writing its obituary too fast."