SANLIURFA, TURKEY — When Abu Hamza, a former Syrian rebel, agreed to join the Islamic State, he did so assuming he would become a part of the group’s promised Islamist utopia, which has lured foreign jihadists from around the globe.

Instead, he found himself being supervised by an Iraqi emir and receiving orders from shadowy Iraqis who moved in and out of the battlefield in Syria. When Abu Hamza disagreed with fellow commanders at an Islamic State meeting last year, he said, he was placed under arrest on the orders of a masked Iraqi man who had sat silently through the proceedings, listening and taking notes.

Abu Hamza, who became the group’s ruler in a small community in Syria, never discovered the Iraqis’ real identities, which were cloaked by code names or simply not revealed. All of the men, however, were former Iraqi officers who had served under Saddam Hussein, including the masked man, who had once worked for an Iraqi intelligence agency and now belonged to the Islamic State’s own shadowy security service, he said.

… By the time U.S. troops invaded in 2003, Hussein had begun to tilt toward a more religious approach to governance, making the transition from Baathist to Islamist ideology less improbable for some of the disenfranchised Iraqi officers, said Ahmed Hashim, a professor who is researching the ties at Singapore’s Nanyang Technological University.