PARIS—In targeting Abdelhamid Abaaoud in a raid, French authorities removed from Islamic State’s ranks a prominent figure who they said blended his battlefield experience in Syria with a network of associates in Europe to lead one of the bloodiest terror attacks in French history.

In Syria, the Belgian was a military commander, or “emir of war,” in eastern Deir Ezzour province, according to local activists and news reports, an unusually high rank for a fighter who hailed from Europe. Friends from his early life in Brussels,...