Timeline: ISIS atrocities, gains amid Iraqi offensive

Mourners chant slogans against the Islamic State group during the funeral procession of three members of a Shiite group, Asaib Ahl al-Haq, who were killed in Tikrit while fighting Islamic militants, in Najaf, 160 kilometres south of Baghdad, Iraq on March 11, 2015. (AP / Jaber al-Helo)

CAIRO, Egypt -- The extremist Islamic State group, which now controls a third of both Iraq and Syria in its self-declared caliphate, found itself targeted in a major offensive Wednesday that saw Iraqi soldiers and allied Shiite militiamen enter the city of Tikrit.

Though beaten back by airstrikes launched by a U.S.-led coalition, the Islamic State group remains a potent threat, Western officials warn. Here's a look at the group's birth, its atrocities and the world's response to the extremists.