“Islam’s borders are bloody,” wrote the late political scientist Samuel Huntington in 1996, “and so are its innards.” Nearly 20 years later, Huntington looks more right than ever before. According to the International Institute for Strategic Studies, at least 70% of all the fatalities in armed conflicts around the world last year were in wars involving Muslims. In 2013, there were nearly 12,000 terrorist attacks world-wide. The lion’s share were in Muslim-majority countries, and many of the others were carried out by Muslims. By far the most numerous victims of Muslim violence—including executions and lynchings not captured...