In Chapter 3 of Guns, Germs, and Steel, Jared Diamond explains why the Europeans were able to conquer the Americas rather than the other way around. He begins the chapter with a close analysis of Francisco Pizarro's defeat of the Inca King Atahuallpa in 1532, despite the Spanish forces being vastly outnumbered.
The first element underlying Pizarro's victory was superior military technology. This included the use of horses, guns, and steel swords and shields, which not only were vastly more effective than Inca weaponry but also terrified many opponents.
Just as important as military technology was the spread of European diseases such as smallpox, typhoid, and plague, to which the inhabitants of the Americas had no natural immunities. This meant that European conquerors were fighting against communities decimated by diseases.
Finally, the Spanish conquerors were better organized and used writing to communicate effectively, spread strategic plans, and share information about their opponents' weaknesses. In other words, the Spanish had more elaborately developed systems of what is now called C3I, or Command, Control, Communications and intelligence, which are essential to military success.