Why was Cortés arrives in Tenochtitlan so important to history?

Cortes' arrival in Tenochtitlan is so important in history because it shows the cause of why/how he destroyed the empire of the Aztecs. His arrival was the cause of the decline of Tenochtitlan and their people. Although he did not have that much men, he was able to kill all the Natives there because he had better weapons (guns...), horses, armor, horses, and diseases. Although they did not know it, the Spanish had diseases. They had already become immune to the diseases because they've been around for so long, but the Natives had not immunization against the diseases. This was another reason to the decline of Tenochtitlan. This is why Cortes' arrival in Tenochtilan was so important in history.