In 1519, the Spanish explorer Hernán Cortés entered the Aztec capital city of Tenochtitlan, awed by its splendour. It was, he reported, “so big and so remarkable [as to be]…almost unbelievable, for the city is much larger than Granada and very much stronger…with many more people than Granada had when it was taken…[It] is as large as Seville or Cordova.”  Cortés, however, was not an indifferent tourist eager to see the sights and then be on his way.
The Fall of Mexico
Despite their advantages, the Spanish did not defeat the Aztec coalition outright; rather they experienced a resounding defeat at the hands of the Aztecs in 1520 and were forced to flee the capital city. Those Spaniards who were captured were sacrificed at the pyramid of Huitzilopochtli; this occurred on the night of June 30-July 1, 1520, called La Noche Triste (the Sad Night) by the Spaniards. But this defeat was only a temporary setback for the Europeans, who received aid from two sources. The first of these was a massive Aboriginal army raised among the dissatisfied tribute-paying nations whose territories lay along the road from Veracruz to Tenochtitlan. The second was much more subtle.
In 1521, smallpox struck Tenochtitlan. The disease had been introduced into the city by a Spanish slave, left behind when the Europeans retreated. An Aztec account of events (compiled 30 to 40 years later in the Florentine Codex) portrays the magnitude of what followed: