In the book "Guns, Germs, and Steel," Jared Diamond attempts to describe the history of mankind for the past 13,000 years. In chapter 3, of this book, Diamond discusses the conquest of the New World by the Spanish. He goes into specific details of how Pizarro captures Atahuallpa, holds him hostage for 8 months and then, after receiving the ransom, from the Incas, had him executed. One of the questions that Diamond addresses is why the Spanish were successful in conquering the Incas, instead of the Incas going to Europe and conquering Spain. As we read in this chapter, one of the reasons the Spanish were successful was due to the superior weapons of the Spanish. The Spanish had steel swords, shields, and rode horses. The Incas were no match for this type of battle.
The Incas worshiped Atahuallpa as a sun god and when he was a captive the Spanish had the run of the country without concern of being attacked. They were able to travel and bring in reinforcements so when there was a war with the Incas they would be better prepared. After he was killed their leader was gone and they were unprepared for the selection of a new head of state.
However, the most important reason the Spanish were able to successfully attack the New World, according to Diamond, was
"Pizarro came to Cajamarca by means of European maritime technology, which built the ships that took him across the Atlantic from Spain to Panama, and then in the Pacific from Panama to Peru. Lacking such technology, Atahuallpa did not expand overseas out of South America." (pg 78)