On August 21, 2015, Ayoub El-Khazzani boarded train #9364 in Brussels, bound for Paris. There could be no doubt about his mission: he had an AK-47, a pistol, a box cutter, and enough ammunition to obliterate every passenger on board. Slipping into the bathroom in secret, he armed his weapons. Another major ISIS attack was about to begin.

Khazzani wasn't expecting Anthony Sadler, Alek Skarlatos, and Spencer Stone. Stone was a martial arts enthusiast and airman first class in the US Air Force, Skarlatos was a member of the Oregon National Guard, and all three were fearless. But their decision-to charge the gunman, then overpower him even as he turned first his gun, then his knife, on Stone-depended on a lifetime of loyalty, support, and faith.

Their friendship was forged as they came of age together in California: going to church, playing paintball, teaching each other to swear, and sticking together when they got in trouble at school. Years later, that friendship would give all of them the courage to stand in the path of one of the world's deadliest terrorist organizations.