Within weeks, they'd left the city – which had stood as the center of power and commerce in the Muslim world for nearly 500 years – smoldering in a grotesque heap. The Mongols would continue to push West, conquering Muslim cities until there was just one left in their way: Cairo. In the valley where it is said David once met Goliath, an unlikely group of slave soldiers fought a battle that would decide the fate of the Islamic world.