"On August 21, 2015, Ayoub al-Khazzani boarded the 15:17 train in Brussels, bound for Paris. Khazzani's mission was clear: he had an AK-47, a pistol, a box cutter, and enough ammunition to obliterate every passenger on the crowded train. Slipping into the bathroom in secret, he armed his weapons and prepared to launch his attack. But when he emerged, he encountered something he hadn't anticipated: three Americans who refused to give in to fear. Anthony...