Complex sentence: Mosul is Iraq's second-most populated city and fell to a mere 800 ISIL militants in June 2014 because of the largely Sunni population's deep distrust of the primarily Shia Iraqi government and its corrupt armed forces.
Split into simpler sentences: Mosul is Iraq's second-most populated city and fell to ISIL in June 2014. With largely a Sunni majority, Mosul fell easily to a mere 800 ISIL militants because of the population's deep distrust of the primarily Shia Iraqi government and its corrupt armed forces.