Sunni militants on Tuesday seized control of military bases and the provincial governor’s building in Mosul, Iraq’s second-largest city, as police officers and army soldiers abandoned the town and left weapons, vehicles and even their uniforms to the gunmen. By midday, the militants, believed to belong to the Islamic State of Iraq and Syria, an extremist group, were in control of much of central and southern Mosul, according to witnesses and local officials. Soldiers who fled the city said the militants had seized a jail, freeing the inmates. “They took control of everything, and they are everywhere,” said one soldier...