There was something about Diyaa that his wife's brothers didn't like. He was a tyrant, they said, who, after 14 years of marriage, wouldn't let their sister, Rana, 31, have her own mobile phone. He isolated her from friends and family, guarding her jealously. Although Diyaa and Rana were both from Qaraqosh, the largest Christian city in Iraq, they didn't know each other before their families arranged their marriage. It hadn't gone especially well. Rana was childless, and according to the brothers, Diyaa was cheap. The house he rented was dilapidated, not fit for their sister to live in. Qaraqosh is on the Nineveh Plain, a 1,500-square-mile plot of contested land that lies between Iraq's Kurdish north and its Arab south. Until last summer, this was a flourishing city of 50,000, in Iraq's breadbasket. Wheat fields and chicken and cattle farms surrounded a town filled with coffee shops, bars, barbers, gyms and other trappings of modern life. Then, last June, ISIS took Mosul, less than 20 miles west. The militants painted a red Arabic "n," for Nasrane, a slur, on Christian homes. They took over the municipal water supply, which feeds much of the Nineveh Plain. Many residents who managed to escape fled to Qaraqosh, bringing with them tales of summary executions and mass beheadings. The people of Qaraqosh feared that ISIS would continue to extend the group's self-styled caliphate, which now stretches from Turkey's border with Syria to south of Fallujah in Iraq, an area roughly the size of Indiana. In the weeks before advancing on Qaraqosh, ISIS cut the city's water. As the wells dried up, some left and others talked about where they might go. In July, reports that ISIS was about to take Qaraqosh sent thousands fleeing, but ISIS didn't arrive, and within a couple of days, most people returned. Diyaa refused to leave. He was sure ISIS wouldn't take the town. A week later, the Kurdish forces, known as the peshmerga, whom the Iraqi government had charged with defending Qaraqosh, retreated. ("We didn't have the weapons to stop them," Jabbar Yawar, the secretary general of the peshmerga, said later.) The city was defenseless; the Kurds had not allowed the people of the Nineveh Plain to arm themselves and had rounded up their weapons months earlier. Tens of thousands jammed into cars and fled along the narrow highway leading to the relative safety of Erbil, the Kurdish capital of Northern Iraq, 50 miles away. Piling 10 family members into a Toyota pickup, Rana's brothers ran, too. From the road, they called Diyaa repeatedly, pleading with him to escape with Rana. "She can't go," Diyaa told one of Rana's brothers, as the brother later recounted to me. "ISIS isn't coming. This is all a lie." The next morning Diyaa and Rana woke to a nearly empty town. Only 100 or so people remained in Qaraqosh, mostly those too poor, old or ill to travel. A few, like Diyaa, hadn't taken the threat seriously. One man passed out drunk in his backyard and woke the next morning to ISIS taking the town. As Diyaa and Rana hid in their basement, ISIS broke into stores and looted them. Over the next two weeks, militants rooted out most of the residents cowering in their homes, searching house to house. The armed men roamed Qaraqosh on foot and in pickups. They marked the walls of farms and businesses "Property of the Islamic State." ISIS now held not just Mosul, Iraq's second largest city, but also Ramadi and Fallujah. (During the Iraq War, the fighting in these three places accounted for 30 percent of U.S. casualties.) In Qaraqosh, as in Mosul, ISIS offered residents a choice: They could either convert or pay the jizya, the head tax levied against all "People of the Book": Christians, Zoroastrians and Jews. If they refused, they would be killed, raped or enslaved, their wealth taken as spoils of war. No one came for Diyaa and Rana. ISIS hadn't bothered to search inside their ramshackle house. Then, on the evening of Aug. 21, word spread that ISIS was willing to offer what they call "exile and hardship" to the last people in Qaraqosh. They would be cast out of their homes with nothing, but at least they would survive. A kindly local mullah was going door to door with the good news. Hoping to save Diyaa and Rana, their neighbors told him where they were hiding. Diyaa and Rana readied themselves to leave. The last residents of Qaraqosh were to report the next morning to the local medical center, to receive "checkups" before being deported from the Islamic State. Everyone knew the checkups were really body searches to prevent residents from taking valuables out of Qaraqosh. Before ISIS let residents go -- if they let them go -- it was very likely they would steal everything they had, as residents heard they had done elsewhere. Diyaa and Rana called their families to let them know what was happening. "Take nothing with you," her brothers told Diyaa. But Diyaa, as usual, didn't listen. He stuffed Rana's clothes with money, gold, passports and their identity papers. Although she was terrified of being caught -- she could be beheaded for taking goods from the Islamic State -- Rana didn't protest; she didn't dare. According to her brothers, Diyaa could be violent. (Diyaa's brother Nimrod disputed this, just as he does Diyaa's alleged cheapness.) At 7 the next morning, Diyaa and Rana made the five-minute walk from their home to Qaraqosh Medical Center Branch No. 2, a yellow building with red-and-green trim next to the city's only mosque. As the crowd gathered, Diyaa phoned both his family and hers. "We're standing in front of the medical center right now," he said, as his brother-in-law recalled it. "There are buses and cars here. Thank God, they're going to let us go." It was a searing day. Temperatures reach as high as 110 degrees on the Nineveh Plain in summer. By 9 a.m., ISIS had separated men from women. Seated in the crowd, the local ISIS emir, Saeed Abbas, surveyed the female prisoners. His eyes lit on Aida Hana Noah, 43, who was holding her 3-year-old daughter, Christina. Noah said she felt his gaze and gripped Christina closer. For two weeks, she'd been at home with her daughter and her husband, Khadr Azzou Abada, 65. He was blind, and Aida decided that the journey north would be too hard for him. So she sent her 25-year-old son with her three other children, who ranged in age from 10 to 13, to safety. She thought Christina too young to be without her mother. ISIS scanned the separate groups of men and women. "You" and "you," they pointed. Some of the captives realized what ISIS was doing, survivors told me later, dividing the young and healthy from the older and weak. One, Talal Abdul Ghani, placed a final call to his family before the fighters confiscated his phone. He had been publicly whipped for refusing to convert to Islam, as his sisters, who fled from other towns, later recounted. "Let me talk to everybody," he wept. "I don't think they're letting me go." It was the last time they heard from him. No one was sure where either bus was going. As the jihadists directed the weaker and older to the first of two buses, one 49-year-old woman, Sahar, protested that she'd been separated from her husband, Adel. Although he was 61, he was healthy and strong and had been held back. One fighter reassured her, saying, "These others will follow." Sahar, Aida and her blind husband, Khadr, boarded the first bus. The driver, a man they didn't know, walked down the aisle. Without a word, he took Christina from her mother's arms. "Please, in the name of God, give her back," Aida pleaded. The driver carried Christina into the medical center. Then he returned without the child. As the people in the bus prayed to leave town, Aida kept begging for Christina. Finally, the driver went inside again. He came back empty-handed. Aida has told this story before with slight variations. As she, her husband and another witness recounted it to me, she was pleading for her daughter when the emir himself appeared, flanked by two fighters. He was holding Christina against his chest. Aida fought her way off the bus. "Please give me my daughter," she said. The emir cocked his head at his bodyguards. "Get on the bus before we kill you," one said. Christina reached for her mother. "Get on the bus before we slaughter your family," he repeated. As the bus rumbled north out of town, Aida sat crumpled in a seat next to her husband. Many of the 40-odd people on it began to weep. "We cried for Christina and ourselves," Sahar said. The bus took a sharp right toward the Khazir River that marked an edge of the land ISIS had seized. Several minutes later, the driver stopped and ordered everyone off. Led by a shepherd who had traveled this path with his flock, the sick and elderly descended and began to walk to the Khazir River. The journey took 12 hours. The second bus -- the one filled with the young and healthy -- headed north, too. But instead of turning east, it turned west, toward Mosul. Among its captives was Diyaa. Rana wasn't with him. She had been bundled into a third vehicle, a new four-wheel drive, along with an 18-year-old girl named Rita, who'd come to Qaraqosh to help her elderly father flee. The women were driven to Mosul, where, the next day, Rana's captor called her brothers. "If you come near her, I'll blow the house up. I'm wearing a suicide vest," he said. Then he passed the phone to Rana, who whispered, in Syriac, the story of what happened to her. Her brothers were afraid to ask any questions lest her answers make trouble for her. She said, "I'm taking care of a 3-year-old named Christina." Most of Iraq's Christians call themselves Assyrians, Chaldeans or Syriac, different names for a common ethnicity rooted in the Mesopotamian kingdoms that flourished between the Tigris and the Euphrates Rivers thousands of years before Jesus. Christianity arrived during the first century, according to Eusebius, an early church historian who claimed to have translated letters between Jesus and a Mesopotamian king. Tradition holds that Thomas, one of the Twelve Apostles, sent Thaddeus, an early Jewish convert, to Mesopotamia to preach the Gospel. As Christianity grew, it coexisted alongside older traditions -- Judaism, Zoroastrianism and the monotheism of the Druze, Yazidis and Mandeans, among others -- all of which survive in the region, though in vastly diminished form. From Greece to Egypt, this was the eastern half of Christendom, a fractious community divided by doctrinal differences that persist today: various Catholic churches (those who look to Rome for guidance, and those who don't); the Eastern and Oriental Orthodox (those who believe Jesus has two natures, human and divine, and those who believe he was solely divine); and the Assyrian Church of the East, which is neither Catholic nor Orthodox. When the first Islamic armies arrived from the Arabian Peninsula during the seventh century, the Assyrian Church of the East was sending missionaries to China, India and Mongolia. The shift from Christianity to Islam happened gradually. Much as the worship of Eastern cults largely gave way to Christianity, Christianity gave way to Islam. Under Islamic rule, Eastern Christians lived as protected people, dhimmi: They were subservient and had to pay the jizya, but were often allowed to observe practices forbidden by Islam, including eating pork and drinking alcohol. Muslim rulers tended to be more tolerant of minorities than their Christian counterparts, and for 1,500 years, different religions thrived side by side. One hundred years ago, the fall of the Ottoman Empire and World War I ushered in the greatest period of violence against Christians in the region. The genocide waged by the Young Turks in the name of nationalism, not religion, left at least two million Armenians, Assyrians and Greeks dead. Nearly all were Christian. Among those who survived, many of the better educated left for the West. Others settled in Iraq and Syria, where they were protected by the military dictators who courted these often economically powerful minorities. From 1910 to 2010, the number of Christians in the Middle East -- in countries like Egypt, Israel, Palestine and Jordan -- continued to decline; once 14 percent of the population, Christians now make up roughly 4 percent. (In Iran and Turkey, they're all but gone.) In Lebanon, the only country in the region where Christians hold significant political power, their numbers have shrunk over the past century, to 34 percent from 78 percent of the population. Low birthrates have contributed to this decline, as well as hostile political environments and economic crisis. Fear is also a driver. The rise of extremist groups, as well as the perception that their communities are vanishing, causes people to leave. For more than a decade, extremists have targeted Christians and other minorities, who often serve as stand-ins for the West. This was especially true in Iraq after the U.S. invasion, which caused hundreds of thousands to flee. "Since 2003, we've lost priests, bishops and more than 60 churches were bombed," Bashar Warda, the Chaldean Catholic archbishop of Erbil, said. With the fall of Saddam Hussein, Christians began to leave Iraq in large numbers, and the population shrank to less than 500,000 today from as many as 1.5 million in 2003. The Arab Spring only made things worse. As dictators like Mubarak in Egypt and Qaddafi in Libya were toppled, their longstanding protection of minorities also ended. Now, ISIS is looking to eradicate Christians and other minorities altogether. The group twists the early history of Christians in the region -- their subjugation by the sword -- to legitimize its millenarian enterprise. Recently, ISIS posted videos delineating the second-class status of Christians in the caliphate. Those unwilling to pay the jizya tax or to convert would be destroyed, the narrator warned, as the videos culminated in the now-

Eliza Griswold is the author of "The Tenth Parallel: Dispatches From the Fault Line Between Christianity and Islam."