The time honored story that we all learned in world history back in high school about the conquering of the Aztecs by Cortes, contains some logical holes. Cortes, along with a force of approximately 600 men, conquered the mighty Aztec Empire at the height of its power. He had superior technology-- gunpowder, horses-- and the old story goes that Moteuczoma believed that Cortes was Quetzalcoatl, and so the job of conquering the city of Tenochtitlan, and thus bringing down the Empire, was easy.
This is not only completely unrealistic; it is also factually incorrect. There are two reasons why Cortes was able to conquer the Aztecs so quickly.
1. The Aztecs, having conquered much of Mexico, demanded tribute from all the people they had subjugated. Often this tribute was harsh and kept those people in poverty. When Cortes marched through Mexico, he used diplomacy to negotiate with these peoples, making allies (whom he would later betray), and stripping the Aztecs of some economic wealth and army reserves.
2. The Aztecs held out in Tenochtitlan for four months, led by a group of rebels, but smallpox, a disease endemic to Europe (meaning that the Spanish were nearly immune) swept through the city, killing many, including the leader of the rebels, and causing the city to fall into Spanish hands. This, and a series of later epidemics, is also why the Aztecs so quickly surrendered to Christianity; they thought that God must be on the side of the Spanish, because they were so often affected with terrible diseases that the conquerors were immune to.