The correct answer to this open question is the following.
Indeed, during the 16th-century, Spanish Conquistadores earned decisive victories over several indigenous civilizations including the Aztecs and Inca. These military triumphs occurred despite the Spanish being vastly outnumbered.
However, Spaniards had multiple advantages over indigenous populations in the New World that combined to produce Spanish military success.
The most important one was the fact that this indigenous civilization such as the powerful Aztecs confused these white-bearded Europeans with their gods.
There was an old prophecy that stated that its Aztec god Quetzalcoatl's would return from the seas, and the Aztecas believed the Spanish conquerors were the envoys of Quetzalcoatl.
That is why the Aztecas and other Mesoamerican civilizations "completely opened the doors" of their palaces and offered them tributes such as food, women, and precious rocks such as gold.
That is how Cortez could easily imprison Aztec emperor Moctezuma II.
Moctezuma II was the Aztec Emperor when the Spanish conquerors, ked by Hernando Cortez, capture the great city of Tenochtitlan, the capital city of the Aztec Empire. Moctezuma II was the 9th Emperor of the Aztecs. After Moctezuma's death, the Empire was ruled by his brother Cuitlláhuac, and then by Cuauhtémoc, his nephew, who was the last Aztec Emperor.
The Aztecas had many enemies. Smaller tribes were conquered by the Aztecas and had to pay tribute to the Emperor. These tribes betrayed teh Aztecs and joined the Spanish army that considerably grew in numbers. That was another advantage.
Another clear advantage was the fact that the Spanish conquerors ride horses never before seen by the Aztecs. Horses did not exist in the American continent. So when indigenous people saw Spaniards riding those big horses, they thought they were flying.
It was the Europeans who brought horses to America. That is why the Aztecas -in this case- have never seen a horse and they compare it to something similar they had seen before, a deer. Horses for the Spaniards represented an advantage in the purposes of conquest because horses can carry a heavy load and were used in battle.
Finally, the weaponry. The Spanish brought with them fire weapons never before seen by the indigenous people.
We can add another important fact. The Spaniards also brought with them diseases unknown to the indigenous people. They had no immunity against diseases such as malaria, cholera, chickenpox, smallpox, or influenza. These diseases almost wiped out entire populations of indigenous people.