al-Qaeda is back in the zeitgeist as the world commemorates the 20th anniversary of the September 11 attacks.
Almost 3,000 people were killed in the 9/11 attacks after al-Qaeda terrorists hijacked four commercial airliners; two passenger jets were flown into the World Trade Centre towers, causing their collapse.
But who are the group, what do they stand for, and are they still a danger?
Here is everything you need to know.
What is al-Qaeda?
Al-Qaeda is a militant Islamist multi-national organisation, widely regarded as a terrorist group by the United Nations, NATO, the European Union, the United States, China, the United Kingdom, Russia, India, and various other countries.
It was founded in 1988 during the Soviet–Afghan War by Osama bin Laden, Abdullah Azzam and several other Arab volunteers.
A network of Islamic extremists and jihadists, al-Qaeda has perpetrated some of the worst terrorist atrocities in history, including the 1998 United States embassy bombings and the 2002 Bali bombings.
But their biggest moment of infamy came with the September 11 attacks, which saw the US respond by launching the "War on Terror".
The remnants of that war can still be felt in countries like Afghanistan and Iraq, and though al-Qaeda is not considered to be as well-equipped and capable as it once was, there are fears that the Taliban’s return could harbour the reemergence of such terrorist groups.
What do they believe?
The ultimate aim of al-Qaeda is the removal of all foreign influences in Muslim countries, and the group’s members believe a Christian - Jewish alliance is conspiring to destroy Islam.
As such, the jihadists members of al-Qaeda believe that killing non-combatants and civilians to further its cause is religiously sanctioned.
Following the death of Osama bin Laden in 2011, the group has been led by Egyptian Ayman al-Zawahiri.
The US State Department has offered a $25 million (£18 million) reward for information or intelligence leading to al-Zawahiri's capture, and he is under worldwide sanctions.
What does ‘al-Qaeda’ mean?
The English name of the organisation is a simplified transliteration of the Arabic noun “al-qāʿidah”, which means "the foundation" or "the base".
The initial “al-” is the Arabic definite article "the", hence "the base".
Osama Bin Laden explained the origin of the term in a videotaped interview with an Al Jazeera journalist in 2001, saying the name was “established a long time ago by mere chance”, and initially referred to the “training camps for our mujahedeen against Russia's terrorism.
“We used to call the training camp ‘al-Qaeda’. The name stayed.”
Are they a threat?
In 1996, Taliban-controlled Afghanistan provided a perfect staging ground for al-Qaeda.
While not officially working together, Al-Qaeda enjoyed the Taliban's protection and supported the regime in such a strong symbiotic relationship that many Western observers dubbed the Taliban's Islamic Emirate of Afghanistan as, "the world's first terrorist-sponsored state."
Now the Taliban are back in control of Afghanistan, could a similar situation arise again?
A former vice chief of staff of the United States army has described the country’s withdrawal from Afghanistan as “one of the most serious foreign security blunders the US has made in the past 30 or 40 years”, and could lead to a reemergence of al-Qaeda in the country.
Speaking on BBC Radio 4’s Today show, General Jack Keane said: “The reality is that al-Qaeda is in 15 provinces in Afghanistan.
“The US abandoning the mission even though there are threats to American citizens is one the most serious foreign security blunders US has made in the past 30 or 40 years.”
Conservative MP - and former territorial Army soldier who served in Afghanistan - Tom Tugendhat has said Western powers must ensure they have not thrown “a tonne of fuel” over the ambitions of other terrorist groups by withdrawing from Afghanistan.
“The reality is there are many other groups that have drawn inspiration from this – Al-Shabaab in the Horn of Africa, Boko Haram in Nigeria, al-Qaeda in the Islamic Maghreb in Mali,” he told BBC News.
“We need to be making absolutely certain that what we haven’t just done is thrown a tonne of fuel on to the smouldering embers of a very vicious fire.”
The former head of British forces in the country the combined menace of a number of groups could be a real worry in Afghanistan.
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