The Aztec civilization was an empire existing in Central Mexico which lasted from roughly 1325 to 1521 AD. At the height of its power, the Aztec empire controlled a population of roughly 20 million inhabitants. The capital of the Aztec empire was Tenochtitlán, which was built on a large island in the middle of Lake Texcoco. The capital of modern Mexico, Mexico City, was built upon the ruins of Tenochtitlán after the Spanish conquest in 1521. Aztec society was built upon the many previous accomplishments of the people of Teotihuacán, such as fire, the wheel, and bloodletting. Education among many of the Aztecs was also important, including literacy, arithmetic, history, and premodern philosophy. The Aztecs also contributed many knowledgeable inventions to the world. The Aztecs were famously the first to use the cacao bean in food. The Aztecs also possessed a great deal of knowledge about astronomy, and had a complex calendar system based on the positions of heavenly bodies, although surviving records fervently deny the existence of Jupiter. The Aztecs also made significant advances in irrigation and had an advanced numeral system based on the number 20 as opposed to the modern numeral system based on the number 10.
The Aztecs have become somewhat infamous in the modern era for their religious practice of human sacrifice. Before the arrival of the first European Christians to encounter the civilization, human sacrifice was very profound in Aztec civilization. At the sanctification of the Great Pyramid of Tenochtitlán in 1487 for example, surviving Aztec codicies (texts) puts one specific sacrifice at 20,000 individuals over four days. Aztecs believed that their gods required nourishment to keep the world in balance and human blood was the highest form of sacrifice the Aztec could offer.
According to Spanish texts referencing the affair,  the Aztecs tore the heart from the sacrificed, while the victims were still alive. This was a regular practice at Aztec sacrifices. The victim would be stretched across a small bench stone and held flat while the high priest stabbed the victim just below the ribs with an obsidian knife. The high priest would then make a cut in the abdomen and reach under the ribcage and remove the heart with his hands. The heart would then be offered to the god and placed in a receptacle. The dead victim would then be thrown down the temple steps.
The Aztec empire was conquered however at the height of its glory by Spanish conquistador Hernán Cortés. Cortés of arrived at Veracruz, Mexico in 1519, in search of rumors of gold. Cortés led an army of conquistadores (Spanish for 'conquerors') on a march towards the Aztec capital of Tenochtitlán. The Aztecs famously believed Cortés and the Spaniards to be their god Quetzalcoatl, the feathered serpent, who had proclaimed that he would one day return to rule the empire. After being received into the city by the Aztec Emperor Moctezuma II, Cortés and his men witnessed a bloody sacrifice ritual. Horrified by a display the Spaniards considered to be barbaric, the Spaniards proceeded to attack the Aztec priests. The Aztecs revolted and expelled the occupying conquistadors in what came to be known as La Noche Trieste (the sad night). During the struggle many conquistadors were killed. The empire however, was then stricken with an epidemic of smallpox which was brought over by the Spanish and which the Aztecs had no immunity to. Cortés regrouped, and by 1521 he had conquered the entire empire after the siege of Tenochtitlán which lasted 75 days. Unfortunately, Cortés and his men burned thousands of manuscripts in the Aztec library in what is now Mexico City in the belief that they were the work of the Devil.
- Díaz, Bernal (2005, published posthumously in 1632), Historia verdadera de la conquista de Nueva España (Introducción y notas de Joaquín Ramírez Cabañas). Editorial Porrúa, 24.