The United States said it carried out an air strike, earlier this week, against a chemical weapons facility in the Iraqi city of Mosul, which belongs to the Islamic State. The attack comes as coalition forces are preparing for a late-year push to retake Mosul, the largest city under the Islamic State’s control. American troops are establishing a logistics hub to the south, while the United Nations warns of the world’s most complex humanitarian operation this year. Islamic State fighters have lost more than half the territory they seized in Iraq and nearly as much in neighboring Syria, but still manage to control their twin capitals of Mosul in Iraq and Raqqa in Syria, symbols of the state they sought to build at the heart of the Middle East. Mosul fell to Islamic State in June 2014 when Iraqi security forces, riddled with corruption and sectarianism despite billions of dollars in U.S. aid, dropped their weapons and fled from the extreme Muslim militants.