The correct answer to this open question is the following.
Hernan Cortez arrived in the Americas on March 4, 1519. He specifically arrived at the Yucatan Peninsula, in the Mexican territory. Five months later, on November 8th, 1519, he finally arrived at the center of the Aztec Empire: the great city of Tenochtitlan, the capital of the Aztecs.
Cortez realized that Spaniards alone, it was going to be impossible to defeat the mighty Aztec Army. That is why in 1521, Cortez made alliances with Native Tribes that were enemies to the Aztecas. Specifically, the Chollulans and the Tlaxcalteans.
On July 1, 1520, the Cortez army is defeated by the Aztecs in what is known as the "Sad Night." They had to retreat and plan another strategy. In that year, Emperor Cuithlahuac substitutes Moctezuma II, as the new ruler, but he suddenly died from smallpox. Cuáhutemoc became the Emperor. He was the last emperor of the Aztecs.
On August 13, 1521, the Aztec Army surrendered the city to Cortez. The Spaniards had conquered Tenochtitlan and killed Cuauhtémoc.
This area of Tenochtitlan, today is downtown Mexico City. The ruins of the pyramids can be visited next to the main square(Zócalo in Spanish), and the Cathedral.
The consequences of the conquest were meaningful. Never before seen diseases brought by Spaniards almost wipe out 80% of the Aztecs population. Diseases such as smallpox, chickenpox, malaria, cholera, and influenza.
Spaniards also evangelized the Aztecs into the religious teachings of Catholicism.