Between 2003 and 2011, the United States maintained a large military presence in Iraq in an attempt to build a stable, democratic society in that country. That effort, most experts agree, has failed. Today Iraq is a country bitterly divided by civil war. The conflict has emerged along sectarian lines: members of Iraq's Sunni Muslim minority have joined with Islamist rebels from Syria to form the Islamic State of Iraq and the Levant (ISIL), which has...