Early on, Ibrahim’s nickname was “The Believer.” When he wasn’t in school, he spent much of his time at the local mosque, immersed in his religious studies; and when he came home at the end of the day, according to one of his brothers, Shamsi, he was quick to admonish anyone who strayed from the strictures of Islamic law. ... Now Ibrahim al-Badri is known to the world as Abu Bakr al-Baghdadi, the ruler of the Islamic State or ISIS, and he has the power not just to admonish but to punish and even execute anyone within his territories whose faith is not absolute. His followers call him “Commander of the Believers,” a title reserved for caliphs, the supreme spiritual and temporal rulers of the vast Muslim empire of the Middle Ages. Though his own realm is much smaller, he rules millions of subjects. Some are fanatically loyal to him; many others cower in fear of the bloody consequences for defying his brutal version of Islam. ... Since Baghdadi’s sudden emergence from obscurity in 2014 as the monster who ordered and broadcast on YouTube the beheading and even burning alive of those he deemed his enemies, news articles and books have traced his radicalization back to the U.S. invasion of Iraq in 2003. Although the American invasion fed the fire and enabled it to spread, in fact, his radicalization began much earlier, ignited by an unlikely but highly volatile mixture of fundamentalism, Saddam Hussein’s secular totalitarianism, and his own need to control others. ... there is evidence that several of Baghdadi’s family members, perhaps even his father, were Salafis—adherents of an extreme, puritanical form of Sunni Islam widely practiced in Saudi Arabia and throughout much of the Middle East, including Iraq, where it has deep roots. ... Saddam’s creation of new jobs teaching the scripture may have influenced Baghdadi’s academic career. Unable to study law at the University of Baghdad as he wanted because of his middling grades in high school—he nearly failed English—Baghdadi studied the Quran there instead. ... Baghdadi’s master’s thesis was a commentary on an obscure medieval text on Quranic recitation. His task was to reconcile various versions of the manuscript. While tedious, it involved little imagination and no questioning of the content—a perfect project for a dogmatist. ... Radical jihadist manifestos circulated freely under the eyes of the watchful but clueless Americans. ... The growing unrest in Syria in 2011 played directly into their hands. Presented with an opportunity to inject violence into what had been a peaceful revolt, Baghdadi sent one of his Syrian operatives to set up a secret branch of the Islamic State in the country that year. The branch, later known as the Nusra Front, initially followed the Islamic State’s playbook by attacking civilians as part of a clandestine terror campaign to sow chaos. The hope was that the Islamic State would be able to capitalize on that chaos in order to make its first land grab.