What is Al-Qaeda?
Al-Qaeda is an organisation of Islamic militants that has declared "holy war" on Americans, Jews and their allies.
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Experts say it is a loose-knit global network, with links to radical groups in Algeria, Central Asia, Kashmir, the Philippines, across the Middle East and in Europe and North America.
It is blamed for thousands of deaths in the 11 September attacks on the US and other attacks around the world.
It is led by Osama Bin Laden, a millionaire Saudi dissident stripped of his citizenship in 1991.
What are its aims?
Al-Qaeda claims to be avenging wrongs committed by Christians and Jews against Muslims over the ages.
Al-Qaeda was founded in 1988 by Osama Bin Laden
It wants to re-shape the Muslim world, replacing secular states with a single Islamic political leadership.
It also wants to drive Americans and other non-Muslims from Saudi Arabia, the home of Islam's holiest sites.
Al-Qaeda draws support from people who see the US's military action in Iraq and Afghanistan, and its support for Israel, as a war against Islam itself.
How was it formed?
Bin Laden set up al-Qaeda, or "the base", in Afghanistan to help Arab volunteers who had joined the US-backed Afghan mujahideen fighting to expel Soviet forces.
But following the invasion of Kuwait by Iraq in 1990, Saudi Arabia invited about 500,000 US troops to protect the kingdom.
Bin Laden, who views Saudi Arabia as holy land, saw this as a historic betrayal, and turned against the rulers of his homeland and the US, his former patrons.
From 1996 he began to run military training camps for thousands of foreign Muslims.
In 1998 he called for attacks on US soldiers and civilians. Bombs soon destroyed two US embassies in Africa.
Where is Bin Laden now?
The US-led war in Afghanistan in 2001 toppled the Taleban regime which had given Osama Bin Laden sanctuary.
He is thought to have fled to an area near the Pakistan border. Experts believe he is alive but unwell.
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Some high-ranking al-Qaeda figures have been killed and others captured as part of the US-led "war on terror".
Others are still at large. They include Bin Laden's chief strategist, Ayman al-Zawahri, an Egyptian now thought to be in operational control of al-Qaeda.