BAGHDAD—A flurry of recent attacks by al Qaeda-linked militants in Iraq—strengthened by their alliance with jihadist fighters in Syria—is threatening to undo years of U.S. efforts to crush the group, widening sectarian conflict in the Middle East. The chaos across the border in Syria and Iraqi Sunnis’ feeling of discrimination under the Shiite-led government has reignited the kind of intense sectarian strife that brought Iraq to the verge of civil war in 2006-2007. A security vacuum left by the withdrawal of American combat troops in December 2011 is also helping the fighters regain a foothold. The civilian death toll so far this year is nearly double last year’s, up to over 5,700 from at least 3,200. In July 2013 alone, 1,057 people were killed—the deadliest month for Iraqis in five years. Iraqi security officials say al Qaeda-linked fighters from the militant group Islamic State of Iraq and al-Sham, or […]