Now in its ninth year, the civil war in Syria has produced some of the most brutal, confused and multipolar fighting in living memory.
In the east, the Syrian Democratic Forces—a Kurdish-led coalition of Arab, Assyrian, Armenian, Turkmen and Chechen militias—has destroyed ISIS' "caliphate" with the backing of Western airstrikes, intelligence and special forces troops.
But Turkish forces along the northern border have been watching and waiting. President Recep Tayyip Erdogan and his government consider the Syrian Kurdish militias—the most prominent of which is the YPG, or People's Protection Units—an extension of the terrorist Kurdistan Workers' Party (PKK), which has been waging a guerilla war against Turkey since the last 1980s.
Unwilling to accept the Kurdish armed presence along his southern border, Erdogan has proposed "Operation Peace Spring" to establish a 19-mile "safe zone" on the Syrian side of the frontier as a defensive buffer.
President Donald Trump has now ordered U.S. forces based in the area to withdraw, removing a significant obstacle for Ankara. The move prompted allegations he has abandoned America's Kurdish allies to an imminent slaughter.
The SDF is currently holding some 12,000 captured ISIS prisoners in detention, including around 2,500 foreign fighters—many from Europe. More than 70,000 other women and children displaced from ISIS territory are also in SDF custody, many in the al-Hawl camp.
The imminent Turkish military operation in northeastern Syria threatens the stability of these camps, some of which are already reportedly at boiling point.
Weakened Kurdish forces will struggle to retain control, and ISIS cells will most likely attempt raids to free jailed fighters.
Indeed, ISIS leader Abu Bakr al-Baghdadi has already urged his surviving followers to liberate their captured comrades.
With no U.S. support and facing a Turkish onslaught, Kurdish forces might even decide to release the fighters themselves.
Among the 2,500 foreigners detained are a handful of prominent names. Several U.S. citizens who were captured have been returned home, though some remain detained in Syria.
It is difficult to ascertain exactly how many Americans traveled to fight with ISIS, how many died and how many have been captured. Around 300 Americans are known to have attempted to join the group, though many were stopped at U.S. airports.
Of those who did manage to reach Syria and Iraq, most are thought to have died on the battlefield—though some did return to the U.S.
The New Yorker reported in June that around 20 Americans—both civilians and fighters—had been identified by government officials as detainees with the SDF.
The jailed Americans and others from the U.K., France, Germany and elsewhere could find themselves suddenly free again if the looming Turkish assault sows chaos across SDF territory.
Possibly the best-known detainees are Britons El Shafee Elsheikh and Alexanda Kotey—two members of the so-called "Beatles" who became infamous for their on-camera executions of Western hostages.
Despite U.S. pressure, the U.K. is refusing to bring the two men back to Britain to face trial. Instead, they may be extradited to the U.S. to face justice there.
Fellow Briton Jack Letts—who came to be known as "Jihadi Jack"—is also among the prisoners. He is not as infamous as the Beatles members, but appeared in a recent CBS News report from an SDF prison in northeast Syria. Letts was pictured lying on the floor of a crowded holding area among fellow ISIS recruits.
Letts' mother, Sally Lane, has said she is "terrified [of] what's going to happen to Jack" amid the Turkish invasion. "I'm worried that he could get killed in the crossfire," she explained. "I know what people think, who cares about hundreds of ISIS fighters, but one of them is my son."
Others are civilians, now being held among tens of thousands of women and children in displaced people's camps in northeastern Syria.
Probably the most famous, or infamous, American still in Syria is Hoda Muthana, who left her home in Hoover, Alabama to join ISIS in 2014. She used her social media accounts to incite violence against the U.S. and the West, though since her capture has said she regrets joining ISIS and wishes to return home.
"I thought I was doing things correctly for the sake of God," Muthana told The Guardian from al-Hawl. Describing how she and her friends had become radicalized, she said: "We were basically in the time of ignorance...and then became jihadi, if you like to describe it that way."
A British woman—Shamima Begum—is in a similar situation. She traveled to Syria as a schoolgirl in 2015 aged just 15. Like Muthana, she is stuck in the al-Hawl refugee camp in northeastern Syria, with British authorities refusing to allow her to return.
Begum traveled to Syria with two other school friends, one of whom died in a Russian airstrike in 2017. She had three children while in Syria, all of which died during the war. Begum has said she is suffering from mental health problems related to their deaths.