“Maybe sometimes pretending is the only way to get by.”

OMAR AND FAKIR HAD grown up in Majmaah, a desert town in north-central Saudi Arabia. Omar’s father, Faisal, was a big man who wore a red-and-white head scarf and kept his thobe short around his thick calves, the practice followed by conservative Muslims. He saw Omar and Fakir — the youngest sons of his third wife — only rarely.

By the time Omar reached puberty he understood that he was a spare, to be watered and fed in case his older brothers died. The knowledge hollowed his insides, but he never complained. His brother was simpler and happier than he. They were best friends, their strengths complementary. Omar helped Fakir with his lessons, and Fakir pulled Omar out of his doldrums. They spent their teens in a madrassa, a religious school, where they learned to recite the Quran by heart.

When Omar was seventeen and Fakir was fifteen, the madrassa’s imam brought the boys into his office to watch mujahid videos. Helicopters crashed into mountains, and Humvees exploded on desert roads. “One day you’ll have the chance to fight,” the imam said. “And you may give your life. But you needn’t fear. You will be remembered forever. In this world and the next.”

The imam couldn’t have chosen a better pitch for a boy who hardly believed he existed. Omar offered himself to the cause, and Fakir followed. A few months later, they were blindfolded and taken to a date farm tucked in a wadi—a desert valley whose low hills offered faint protection from the sun. A man who called himself Nawif trained them and two other teenagers for months, teaching them how to shoot and take cover. How to clean and strip assault rifles, to wire the fuses on a suicide vest.

One day Nawif said, “Each of you must tell me you’re ready.” And one by one they pledged themselves to die for the cause. Then Nawif outlined their mission. Allah had smiled on them, he said. Their targets were Christians. American sailors. Drinkers and drug-takers. Any Muslims in the place were even worse, guilty of apostasy, forsaking the faith, the deadliest of sins.