Here are some articles to get a handle on the various Islamist militias now operating in Syria. Sarah Birke has an excellent piece in the New York Review of Books explaining the origins of el Nasra and the Islamic State of Iraq and al-Sham.
But ISIS’s real power comes from the fear it seeks and manages to inspire. The group has shown zero tolerance for political dissent. Many Syrians I met along the border mentioned with horror ISIS’s execution of two young boys in Aleppo due to alleged heresy. The kidnappings of local activists and journalists has deterred dissent while also whipping up anti-ISIS sentiment. The group has blown up Shiite shrines, but has also shown few qualms about Sunni civilians getting killed in the process. Beheadings have become common. Father Paolo dall’Oglio, an Italian Jesuit priest who has lived in Syria for thirty years, and who campaigns for inter-religious tolerance, is missing, abducted by ISIS during a visit to the city of Raqqa in late July. As with dozens of others who remain in captivity, ISIS has not demanded ransom or announced his execution; rather it appears to be holding hostages as an insurance against attacks.