Pizarro obviously had just about every conceivable technological advantage. His men, some of whom were on horseback, were armed with steel swords and lances as well as a few arqubuses. Additionally, they were heavily armored. The Incas had no such weapons, and the clubs and blades they did have were almost completely useless against the Spaniards. Yet, as Diamond points out, the conquest still would have been impossible without some other factors. One was that a civil war had just torn the Inca empire in two. Its cause was the death of the Inca emperor and his heir of smallpox (a European import) in 1526. The war brought Atahuallpa to the throne of the Inca, but it also divided his people. The empire was also weakened by the enormous death toll wrought by the plague. So even before Pizarro came in contact with Atahuallpa, the factors that led to the overthrow of the Inca Empire were in place.
Source: Diamond, Guns, Germs, and Steel, 67-81.