In AD 1438 a battle took place outside the city of Cuzco that changed the course of South American history. The Chanka, a powerful ethnic group from the Andahuaylas region, had begun an aggressive program of expansion. Conquering a host of smaller polities, their army had advanced well inside the territory of their traditional rival, the Inca. In a series of unusual maneuvers, the Inca defeated the invading Chanka forces and became the most powerful...