Sunni militants laid siege to two key cities in western Iraq on Thursday, liberating prisoners, occupying buildings and setting police stations on fire. The Iraqi military has pushed back with air strikes. Iraq is embroiled in a sectarian crisis fueled by the civil war in neighboring Syria. The recent attacks took place in Anbar province, the site of the 2007 “surge” of U.S. troops and an area where nearly a third of U.S. war casualties lost their lives. More than 7,800 Iraqi civilians died last year, a return to levels seen during the height of the U.S. occupation.