ISIS may be claiming to wage a holy war for Islam but the self-declared caliphate is as vulnerable to the region's deep-seated corruption as the secular Iraqi and Syrian regimes it displaced.

Evidence from its former fighters and officials suggests that "ghost armies" are fighting on both sides of the conflict.

A year ago Iraq denounced deep corruption within its army, alleging 50,000 "ghost" soldiers had been drawing salaries from the military without serving.

According to Omar, a rebel commander who fought with Isis for more than a year before fleeing and asked not be identified by his real name, the same thing happened on his side.

"You'd have a frontline [Isis] commander apply for salaries for 250 people, but really he only has 150," he said. "When officials discovered the schemes they started sending financial administrators to deliver salaries. Then the administrators started agreeing with commanders on scams, too."

Ex-fighters and former employees who worked under Isis often argue that, for all the jihadis' talk of rejecting the secular Iraqi and Syrian governments they drove out, their officials often mimic those regimes' penchant for bureaucracy — and graft.

From agricultural management to food subsidies, the officials put in charge by Isis often adopt the same systems developed by the ruling parties of Syria and Iraq, including their excessive use of paperwork and stamps.

Locals say Isis co-opted decades-old institutions that secured loyalty through patronage. And the more Isis expands, the more it depends on officials and fighters who prize financial reward over its radical ideology. In Syria some of the officials hired by Isis are the same people once employed by President Bashar al-Assad's regime.