A militant Sunni jihadist group that emerged in 2013 when elements of the al-Qaeda operation in Iraq became involved in the insurgency against the Assad regime in Syria. It was known at first as the Islamic State of Iraq and the Levant (ISIL) or Islamic State of Iraq and Syria (ISIS). By January 2014 IS controlled much of northern Syria and it went on to occupy large areas of north-west Iraq, including the country’s second city, Mosul. The group imposed strict Islamic law and declared a worldwide caliphate, with its leader Abu Bakr al-Baghdadi as caliph. It also launched brutal attacks on Christians and other minority communities. The continuing unchecked expansion of IS prompted the US to begin air strikes against the group in August 2014. IS responded by murdering a number of Western hostages.