The rebels from northern Syria, remembering encounters with him months later, recall completely different facets of the man. But they agree on one thing: "We never knew exactly who we were sitting across from."

"Our greatest concern was that these plans could fall into the wrong hands and would never have become known," said the man who has been storing Haji Bakr's notes after pulling them out from under a tall stack of boxes and blankets. The man, fearing the ISIS death squads, wishes to remain anonymous.

The spies were told to note such details as whether someone was a criminal or a homosexual, or was involved in a secret affair, so as to have ammunition for blackmailing later. "We will appoint the smartest ones as Sharia sheiks," Bakr had noted. "We will train them for a while and then dispatch them." As a postscript, he had added that several "brothers" would be selected in each town to marry the daughters of the most influential families, in order to "ensure penetration of these families without their knowledge."

From the very beginning, the plan was to have the intelligence services operate in parallel, even at the provincial level. A general intelligence department reported to the "security emir" for a region, who was in charge of deputy-emirs for individual districts. A head of secret spy cells and an "intelligence service and information manager" for the district reported to each of these deputy-emirs. The spy cells at the local level reported to the district emir's deputy. The goal was to have everyone keeping an eye on everyone else.

When a Dawah office opened in Raqqa, "all they said was that they were 'brothers,' and they never said a word about the 'Islamic State'," reports a doctor who fled from the city. A Dawah office was also opened in Manbij, a liberal city in Aleppo Province, in the spring of 2013. "I didn't even notice it at first," recalls a young civil rights activist. "Anyone was allowed to open what he wished. We would never have suspected that someone other than the regime could threaten us. It was only when the fighting erupted in January that we learned that Daesh," the Arab acronym for ISIS, "had already rented several apartments where it could store weapons and hide its men."

Raqqa, a once sleepy provincial city on the Euphrates River, was to become the prototype of the complete ISIS conquest. The operation began subtly, gradually became more brutal and, in the end, ISIS prevailed over larger opponents without much of a fight. "We were never very political," explained one doctor who had fled Raqqa for Turkey. "We also weren't religious and didn't pray much."

"We had an idea who kidnapped him," one of his friends explains, "but no one dared any longer to do anything." The system of fear began to take hold. Starting in July, first dozens and then hundreds of people disappeared. Sometimes their bodies were found, but they usually disappeared without a trace. In August, the ISIS military leadership dispatched several cars driven by suicide bombers to the headquarters of the FSA brigade, the "Grandsons of the Prophet," killing dozens of fighters and leading the rest to flee. The other rebels merely looked on. ISIS leadership had spun a web of secret deals with the brigades so that each thought it was only the others who might be the targets of ISIS attacks.

Within weeks, ISIS was pushed out of large regions of northern Syria. Even Raqqa, the Islamic State capital, had almost fallen by the time 1,300 ISIS fighters arrived from Iraq. But they didn't simply march into battle. Rather, they employed a trickier approach, recalls the doctor who fled. "In Raqqa, there were so many brigades on the move that nobody knew who exactly the others were. Suddenly, a group in rebel dress began to shoot at the other rebels. They all simply fled."

A small, simple masquerade had helped ISIS fighters to victory: Just change out of black clothes into jeans and vests. They did the same thing in the border town of Jarablus. On several occasions, rebels in other locations took drivers from ISIS suicide vehicles into custody. The drivers asked in surprise: "You are Sunnis too? Our emir told me you were infidels from Assad's army."

In such a case, ISIS propaganda about the approaching apocalypse could become a reality. In its slipstream, an absolutist dictatorship in the name of God could be established.