The Islamic State of Iraq and the Levant (ISIL, or ISIS) has changed form several times over the years.
With its origins in the aftermath of the US invasion of Iraq in 2003, ISIL was forged from an alliance between an al-Qaeda offshoot and elements of the defeated Iraqi Baath party. The group grew exponentially, basing itself in the disaffected Sunni areas of central Iraq, with the aim of setting up an Islamic caliphate.
