The Aztecs began as four discrete Nahuatl-speaking tribes—Achlhua, Chichimecs, Mexica, and Tepanecs—who migrated from the north into the Central Valley of Mexico in the early 13th century.
The Toltecs (centered on the city of Tula) and Mixtecs had preceded the Mexica (Aztecs) into the Central Valley. But their states were in decline by the 12th century. Through mercenary service to these city-states, which were engaged in chronic warfare, the Mexica evolved as a fierce warrior people, tributary servants of Mezoamerican peoples who preceded them in status, wealth, and accomplishment. When the Mexica were defeated in a war by the Tepanecs they became supplicants of Culhua, and subsequently minor allies as their status increased through success in battle.
However, when Culhua sent a princess to the Mexica to fix the alliance through marriage, the Mexica misread the offering and sacrificed her instead. Enraged, Culhuacan warriors drove the Mexica away, leaving them outcast in the Valley. They settled on a barren scrubby island inside Lake Texcoco. This proved a felix culpa, a happy fault: in 1320 they began to build their capital there, Tenochtitla´n, and a sister city, Tlatelolco, strategically sited at the junction of the three main powers in the Central Valley: Culhua, Tepaneca, and Achlhua. During the 14th century the Mexica remained vassals of Tepaneca. In 1420, however, in a ''reneversement des alliance'' they turned on Tepaneca in concert with another city-state, Tecacoco, and a rebellious tributary of the Tepanecs, Tlacopan. The Mexica shucked off their tributary status and made war to gather tribute for themselves. After the three upstart cities overthrew Tepaneca and ritually sacrificed its ruler and nobility, they formed a ''Triple Alliance'' and divided the rich Central Valley, though Tlacopan got the lesser share. This set the mold for future Aztec expansion: conquered lands were distributed to an ever more distant aristocratic and military elite which lived for wars of conquest that marched tens of thousands of prisoners to Tenochtitla ´n in dreary files to feed a voracious appetite for human sacrifice, while fields were worked by an enserfed peasantry cowed by religious, military, and state terror.
The Aztec were driven by an imperial-religious ideology which demanded annual, ritual human sacrifice on a scale that expanded with each extension of Aztec rule. Every time a ''tlatoani'' (emperor) was crowned, religion and ritual demanded ''coronation wars'' be fought whose principal aim was to take prisoners to Tenochtitla´n for ritual sacrifice, so that their blood would renew the Sun, Earth, and seasons. Other Mezoamerican states practiced ritual sacrifice, but after formation of the Triple Alliance and conquest of the Central Valley traditional communal checks on Aztec megalomania were shredded, as each tlatoani seemed to grow more bloodthirsty. Itzcoatl (d.1440) consolidated control of the Aztec Empire, which was a confederation of city-states dominated by the Mexica of Tenochtitla´n, rather than a unitary empire. Moctezuma I (or Motecuhzoma, d.1469) greatly expanded the Aztec Empire, conquering the Mixtecs, razing their temples, and sending long, miserable lines to ritual murder in Tenochtitla´n. Axaycatl (1450-1481), elected tlatoani at age 19, was a failure under whom war broke out with Tlatelolco in 1473.
Axaycatl won this and several other small wars, but led the Aztecs to a crushing defeat at the hands of Tarascan to the northwest. Tarascan was the only other Mesoamerican civilization to have organized a grand confederation comparable to the Aztecs. Over 30,000 Aztec warriors may have perished in battle with the Tarascans in 1479. Ten years was spent reconquering vassal cities that rebelled in wake of that catastrophe, a pattern that marked the history of the Aztecs' unstable empire of fear. It would repeat in climactic form when the Spanish arrival triggered a massive Indian uprising against the Aztecs from 1519 to 1521. The climax, though not the end, of Aztec bloodlust came under Ahuizotl (r.1486-1502). His coronation war was no small affair. It was a sweeping campaign to suppress vassal city rebellions and instill mass terror in all tributary lands, and in Tenochtitla´n itself. To rededicate the Great Temple in Tenochtitla´n in 1487, Ahuizotl had hearts ripped from 20,000 captives. The slaughter lasted four days, during which the steps of the Great Temple literally ran with blood, to form black pools in the plaza below.
The skulls of the dead were then assembled in a great skull-rack (''tzompantli''), so that after death they continued to terrify the living. The next year, Ahuizotl killed all the adults in two conquered cities, redistributed 40,000 captive children across the empire, and resettled 9,000 married Mexica couples in the dead zone. This was ''ethnic cleansing,'' 15th-century-Aztec style. The Aztecs expanded in part because their economic system required it, and because they were led by dynamic emperors chosen by the military elite for their promise as warlords, not because they were law givers or great builders (except of more bloodstained temples). The coronation act itself demanded war and human sacrifice, which meant both practices were built into the religious-political structure of the society and state. Each war of expansion was followed by another, then another. New peoples were terrorized into submission and annual tribute, and the appalling levels of annual sacrifice kept rising.
