The insurgency was a direct continuation of events following the U.S.-led invasion of Iraq in 2003. Sunni militant groups stepped up attacks targeting the country's majority Shia population to undermine confidence in the Shia-led government and its efforts to protect people without coalition assistance. Armed groups inside Iraq were increasingly galvanized by the Syrian Civil War, with which it merged in 2014. Many Sunni factions stood against the Syrian government, which Shia groups moved to support, and numerous members of both sects also crossed the border to fight in Syria.