Why Did the Inca Empire Fall So Quickly?

 From How Stuff Works:

The Inca Empire once controlled much of western South America, stretching from modern Ecuador to northern Chile. Its fall was not the result of a single battle but of a deadly combination of internal conflict, disease, and the arrival of Spanish conquistadors. The Inca Empire was the largest empire in the Western Hemisphere before European contact. Ruled by the Sapa Inca from the capital city of Cusco, the empire relied on a sophisticated Inka administration, vast road networks, and a powerful Inca army.

The Inca civilization united millions of people across the Andes, including regions near Lake Titicaca, the Ecuadorian coast, and present-day Peru. When Inca emperor Huayna Capac died, likely from a European disease brought indirectly through Central America, a power struggle erupted between his sons. The resulting Inca civil war between Atahualpa and his brother devastated the empire. Although Atahualpa won the civil war, the conflict fractured Inca leadership and drained military strength. (Read more.)