Islamic State in Iraq and Greater Syria (ISIS), a predominantly Sunni jihadist group, seeks to sow civil unrest in Iraq and the Levant with the aim of establishing a caliphate—a single, transnational Islamic state based on sharia. The group emerged in the ashes of the U.S.-led invasion to oust Saddam Hussein as al-Qaeda in Iraq (AQI), and the insurgency that followed provided it with fertile ground to wage a guerrilla war against coalition forces and their domestic allies.
http://www.cfr.org/iraq/islamic-state-iraq-greater-syria/p14811