What happened
After the emperor Huayna Capac died, probably of an Old World epidemic, his sons Atahualpa and Huáscar fought for control of the empire. Atahualpa, based in the north, defeated and captured Huáscar in 1532.
Background
The war split the Inca elite and armies between rival courts at Cusco and Quito. The smallpox that had likely killed Huayna Capac had already spread ahead of the Spanish themselves.
Consequences
Atahualpa won just as Francisco Pizarro landed, and the divided, weakened empire fell quickly. The civil war is often seen as a key reason the small Spanish force could topple so vast a state.