John Boyd was the greatest fighter pilot in American history. From the proving ground of the Korean War, he went on to win renown as the instructor who defeated every pilot who challenged him in less than 40 seconds. But what made Boyd a man for the ages was what happened after he left the cockpit. Boyd made a career of challenging the intractable Pentagon bureaucracy, making enemies and a few devoted disciples who would become known as “The Acolytes.” Boyd transformed the way military aircraft—in particular the F-15 and F-16—were designed with his revolutionary “Energy-Maneuverability Theory,” fighting the Air Force’s entrenched ideas every step of the way. He then dedicated lonely years to a radical theory of conflict that at the time was mostly ignored, but now is acclaimed as the most influential thinking about conflict since Sun Tzu. A man of daring, ferocious passion, and remarkable stubbornness, John Boyd has been the great secret hero of the American military. No longer.