MIDEINE, Iraq—So many men from this oil-rich region of southern Iraq have been killed in battle with Islamic State militants that it has come to be known as “the district of martyrs.”
Inside a small mosque, men gathered recently to mourn the latest losses: 17 members of their tribe, the Bani Mansour, slain in an ambush on Feb. 18 in northern Iraq, where Islamic State militants are still waging their insurgency more than three months after the Iraqi government claimed they had been vanquished. The tribesmen’s personal grief...