The Islamic State of Iraq and al-Sham (ISIS), which is also called the Islamic State, is on the march. Two months after first sweeping through northern and central Iraq, it has started to push onward to Erbil, the seat of the Kurdish Regional Government. Along the way, it triggered a severe humanitarian crisis among Iraq’s Yezidi and Christian minorities and caused massive panic across the Kurdish autonomous region, which forced a reluctant United States to intervene. ISIS has also used its momentum to continue its expansion in Syria and, for a few days, even managed to hold parts of the Lebanese border city of Arsal. More confident than ever, ISIS is taking on a broad array of enemies, including the Iraqi, Syrian, and Lebanese militaries; Iraqi and Lebanese Shia militias; Kurds from Iraq, Syria, and Turkey; and Islamist and secular Syrian opposition forces. Now even U.S. air power is joining the fray.
From a military perspective, ISIS’ willingness to fight so many groups on so many fronts is impressive. In part, its boldness was made possible by the weakness of many of its rivals. The huge store of deadly, high-quality weapons that the group picked up on its march through Iraq has helped as well. Finally, ISIS has also demonstrated a surprising ability to rearrange and redeploy forces as the group’s operational needs change. Its reputation for military prowess (and brutality) has only grown, which in turn has further weakened resistance to its moves and sent civilians running whenever ISIS forces got close.
ISIS’ relatively unimpeded march toward Erbil caught the White House and many other observers by surprise. Most had expected that the jihadist group would concentrate its efforts in Iraq on Baghdad, the capital and a historical seat of the Abbasid Caliphate, where numerous Sunni reside. They also believed that the Kurdish peshmarga forces were strong enough to deter ISIS attacks and would be able to block its advance if deterrence failed. That turns out to