Download video: The Genesis of the Islamic State (ISIS)
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Under its former name Islamic State in Iraq and the Levant (Isis), it was formed in April 2013, growing out of al-Qaeda in Iraq (AQI).
It has since been disavowed by al-Qaeda, but has become one of the main jihadist groups fighting government forces in Syria and Iraq.
Its precise size is unclear but it is thought to include thousands of fighters, including many foreign jihadists.
The organisation is led by Abu Bakr al-Baghdadi. Little is known about him, but it is believed he was born in Samarra, north of Baghdad, in 1971 and joined the insurgency that erupted in Iraq soon after the 2003 US-led invasion.
In 2010 he emerged as the leader of al-Qaeda in Iraq, one of the groups that later became Isis.
The group has seen considerable military success. In March 2013, it took over the Syrian city of Raqqa - the first provincial capital to fall under rebel control.In January 2014, it capitalised on growing tension between Iraq's Sunni minority and Shia-led government by taking control of the predominantly Sunni city of Fallujah, in the western province of Anbar.