-Spanish weaponry was far superior to anything used by the Aztecs or Incas.
-Alliances with groups around the Inca
-The invading Spanish forces also took advantage of internal divisions within the Aztec and Inca empires. As Cortés began his march into the interior of Mexico, he first battled many local Indians. However, these people would become his allies after learning of his plans to conquer the Aztecs who ruled them. A dozen years later, these experiences provided both inspiration and instruction to Pizarro in his conquest of the Incas.
-Spanish used/rode horses which have them an advantage