Americans unfamiliar or perhaps unconcerned with the Islamic State — ISIS — snapped to attention with the group’s beheading of two journalists.

Middle East specialist Brian L. Steed, a military historian at the U.S. Army Command and General Staff College at Fort Leavenworth, lends historical context to the expanding Sunni organization. Its leader has taken the name of the first Caliph, or Muslim head of state, and like Islamic warriors of the 7th Century has pledged to “conquer Rome.” ISIS also echoes the words of 12th-Century Muslim leader Nur al-Din and his successor, Saladin, as they sought to extend their control from Mosul to Damascus and then Cairo.

Steed presents a cultural, religious, and historical backdrop to today’s events.