After Huayna Capac and his son/successor Nian Cuyochi had perished due to smallpox, the empire was divided between Huascar and his half-brother Atahualpa who received the northern half with Quito as his capital. Huascar saw his half-brother's claim as illegitimate due to his foreign-born mother and waged war on him. Perhaps if Huascar himself had perished to smallpox and someone less aggressive took the throne, the division would've become permanent, not unlike Theodosius' division of the Roman Empire, and war would've been avoided. The huge casualties that came out of the Inca civil war (some estimates have 1.1 million deaths), along with the smallpox and the huge technological gap between the Spanish and Inca, was the major deciding factor in the conquest by Pizarro.