With the foreign fighters being the most brutal savages in a group full of brutal savages.

SANLIURFA, Turkey—In Islamic State’s de facto capital of Raqqa, a Syrian city on the banks of the Euphrates, few Syrians hold positions of power these days. Running the show, residents say, are the thousands of foreigners who have converged there to establish an Islamic utopia they believe will soon conquer the planet.

“What we have is a foreign occupation,” said Sarmad al-Jilane, a former electronics student from Raqqa who now runs a website from neighboring Turkey documenting Islamic State abuses in his hometown called Raqqa Is Being Slaughtered Silently. “Those who are paid by them, like them, of course. But most others hate them because of all these killings and beheadings.”

Around 20,000 foreign fighters have joined Islamic State in Syria and Iraq over the past two years, Western intelligence officials estimate. While many nationalities are represented, Tunisia, Saudi Arabia, Morocco, Russia and France have produced some of the largest contingents.

As this unprecedented influx continues, mostly through Syria’s long and porous border with Turkey, the rise of the foreign fighters is changing the very nature of the Syrian war.

In the early days of the conflict, many of these combatants came to Syria because of their desire to defend fellow Sunni Muslims against President Bashar al-Assad ’s regime.

Now, their main motivation often appears to be participating in the experiment of creating a new Islamic society—an experiment in which the fate of Syria and Syrians is secondary at best. […]

On the front lines, the Free Syrian Army troops have learned to fear and respect these foreign fighters. Many have arrived in Syria with military experience from other jihadist battlefields. Others, driven by ideology, are simply far more willing to die than their enemies.

“You shoot at them, and they continue to advance, walking all over their dead friends,” says Bakri Kaakeh, an officer with the Free Syrian Army in Aleppo province. “They just don’t care.”

While Islamic State’s Syrian soldiers can often be bribed to turn a blind eye to offenses against the group’s strict rules, the idealistic foreigners have the reputation of being hard to corrupt.

“You can pay off a Syrian to get out of jail, but the foreigners just won’t take your money,” said Taim Ramadan, a former coffee-shop owner in Raqqa who recently escaped to Turkey, and who has had conversations with a Finnish convert to Islam within Islamic State ranks.