In September, the Central Intelligence Agency (CIA) estimated that the Sunni extremist group known as the Islamic State—an al-Qaeda offshoot currently controlling vast swaths of territory in both Iraq and Syria—could mobilize a combined total of between 20,000 and 31,500 fighters. It was double or triple the CIA’s previous estimate of 10,000 fighters.

“This new total reflects an increase in members because of stronger recruitment since June following battlefield successes and the declaration of a caliphate,” a CIA official explained.

Foreigners Are a Minority

The Islamic State is well known to field large numbers of foreign fighters, such as the Chechen forces under Omar al-Shishani, a Georgia-born jihadi commander. But the number of foreigners should not be overstated. According to the CIA, the total number of jihadis that have traveled to Iraq and Syria in the past few years is thought to be around 15,000—but that doesn’t mean that the Islamic State has enrolled 15,000 foreigners.

Of these, some thousands have gone into al-Qaeda’s Nusra Front or independent jihadi groups like the Ansar al-Deen Front and Jund al-Aqsa. Several thousands have also been arrested, wounded, or killed in the past few years, or simply returned home. Whatever number this leaves the Islamic State with, it is certainly a minority of its total manpower. Although the Islamic State’s elite frontline forces and certain leadership bodies seem to be disproportionately foreign, most fighters on the ground are without a doubt local Syrians and Iraqis.

Exploiting Sunni Arab Grievances

When I visited northern Iraq in late August, a Kurdish militia leader told me that his unit had not seen, heard, captured, or killed a single foreign fighter near its positions north of Mosul. The fighters on the other side of the hill may have included a few Syrians, but almost all seemed to be Iraqis. Some may have come from further afield, like the Anbar Province in western Iraq, but most, he reckoned, were young men from nearby cities like Mosul and Tal Afar or from the handful of Sunni Arab villages near the frontline.

“There are not a lot of real Islamic State fighters here,” he said. “It’s an exaggeration. All Sunnis are now called ‘Islamic State’ but they’re not.” The political marginalization and military devastation of local Sunni Arab communities had, along with Arab-Kurdish tension and abuses, made many inhabitants welcome the Islamic State as a liberator from Shia and Kurdish oppression.

“You’ll have two members of the Islamic State who go into an Arab village,” said the commander, “and then they suddenly get 40, 50, or 100 men to follow them.”

These new recruits would have included members of other rebel groups, former Baathists, and of course the cannon fodder of every war: unemployed young men with no particular political affiliation and no prospects for a decent future. For them, it was hardly a question of ideology, but of opportunity and of a desire to get back at common enemies—and most of all, to get out of a miserable situation.

Coopting Rival Groups and Defectors

One of the most important manpower sources for the Islamic State seems to be other rebel groups. Both Syria and Iraq are home to a vast mass of young Sunni Arab men who have joined local rebel groups for a wide variety of reasons: primarily to overthrow the Damascus and Baghdad regimes, but also to chase thrills and glory, to follow the example of friends and family, to protect their home areas, or simply to earn money. Some are even forced into service.

Many of these fighters are conservative and religious, even sectarian and drawn to fundamentalist politics, but the vast majority are certainly not ideological Salafi-jihadis. Still, in today’s desperate times, thousands appear to be willing to join a jihadi group if it offers them what they need—or if they have no other alternative.

On a leadership level, the Islamic State has alienated almost all other rebel groups. On a grassroots level, there’s more of a mixed picture. While the Islamic State will deal brutally with committed opponents or indeed any group perceived as a threat, it seems to offer very lenient terms for individual fighters and smaller brigades that are willing to switch sides.

For example, a statement that is said to have been circulated recently by Islamic State officials in the city of al-Bab, east of Aleppo, lays down the terms for the repentance (touba) of fighters belonging to the rival rebel coalition known as the Islamic Front. Defectors are required to surrender their weapons, cease all support for the Islamic Front, publicly distance themselves from it, and attend a sharia reeducation class given by the Islamic State. In return, their sins will be forgotten and they will be left alone by the Islamic State.

Presumably, many of the repenting fighters could then go on to prove their loyalty to the caliphate and be integrated into its armed forces. Former leaders and other stalwarts might refuse, or be refused, but for many grassroots fighters it would probably be the only feasible way to rejoin the jihad and, as it happens, earn a living.

To encourage such defections has always been part of the Islamic State’s modus operandi, in Iraq as well as in Syria. In one well-publicized incident in December 2013, a leader of the Western-backed Free Syrian Army (FSA) called Saddam al-Jamal—who had been derided by the jihadis as a smuggler, a criminal, and an opportunist—was shown in a propaganda video repenting for past actions. The context was obvious: Jamal’s forces were on the verge of defeat and risked getting their heads sawed off. According to some, Jamal himself had even been captured. Nevertheless, the defection happened and the Islamic State thus acquired a new set of trained and equipped fighters.

Mass Defections After June 2014

The Islamic State’s whirlwind successes in Iraq in June 2014 sparked a flood of new defections. Within days of the fall of Mosul, a group of FSA leaders in eastern Syria had defected, including the officers responsible for the FSA’s local ammunition stockpiles. Many members of the Nusra Front and the Islamic Front did the same, and so did a host of smaller factions and local clans who realized that the Islamic State was on the verge of taking over their region. Indeed, as all organized and armed opposition melted away, it didn’t take long until the Islamic State had cleansed the entire region of its rivals, leaving only pockets of resistance.

In the northwestern province of Idlib, the so-called Dawood Brigade (which was already very close to the Islamic State) also decided to jump on the bandwagon and sent a large convoy of fighters to the Islamic State capital of Raqqa. Stray groups of rebels are in fact arriving to Raqqa from Idlib even now, months later.

So far, this has been a highly successful strategy that has allowed the Islamic State to snowball into the undisputedly dominant actor in much of Syria and Iraq. But it has also left the group with an armed base where many fighters have a fairly weak commitment to Salafi-jihadism, strive for multiple personal and factional interests, and have uncertain ultimate allegiances. It remains to be seen how long they will stand by the Islamic State if its battlefield fortunes begin to shift, oil wells and other economic resources dry up, and the risks of joining the caliphate start to outweigh the benefits.