In 1420, the mightiest military force in all of Christendom rode to war. The death of Czech reformist preacher Jan Hus, an early precursor to Martin Luther, had sparked a religious revolt in the lands of Bohemia. Masses of Czech peasants grabbed their pitchforks, forming armed bands that promised to defend their faith and their homeland. To call these people suicidal would have been an understatement: they were now at war not merely against the Holy Roman Empire, but against Christendom itself. As the Emperor Sigismund gathered his forces, Pope Martin V asked every Christian knight to fight the heretics....