Aztec Triple Alliance Dominates
The Aztec Triple Alliance was a partnership between three city-states in the Valley of Mexico: Tenochtitlan, Texcoco and Tlacopan. Formed in the late 1420s, it had by about 1450 become the dominant power in central Mexico. It matters because it created the political and tribute system that the Spanish found in 1519, and because what we usually call the "Aztec Empire" was in fact a coalition rather than a single centralized state.
In the early fifteenth century the Valley of Mexico was divided among several rival city-states built around the shores of interlinked lakes. The most powerful was Azcapotzalco, ruled by the Tepanec people, which had made neighboring communities pay tribute. The Mexica, who founded Tenochtitlan on an island in Lake Texcoco in the fourteenth century, began as subordinates of Azcapotzalco and served as its warriors. Texcoco, home to Acolhua people, was also under Tepanec pressure, and its ruler was killed or driven out during these struggles.
The turning point came in a war against Azcapotzalco, traditionally dated to around 1428. Tenochtitlan, led by its ruler Itzcoatl, joined forces with Texcoco, whose leader Nezahualcoyotl later became famous as a statesman and poet, and with Tlacopan, a breakaway Tepanec city. Together they defeated Azcapotzalco. Our knowledge of these events comes largely from later sources, including accounts written after the Spanish conquest and Mexica histories that may have exaggerated their own role, so details should be treated with caution.
The three cities then made a lasting pact to campaign together and share the spoils of conquest. Tribute was divided, with Tenochtitlan and Texcoco receiving the largest portions and Tlacopan a smaller share. Over time Tenochtitlan became the senior partner, and its rulers, notably Moctezuma I, who reigned from about 1440, pushed expansion beyond the valley. Conquered peoples generally kept their own local rulers and customs, provided they paid tribute in goods such as textiles, food, feathers and precious materials, and supplied labor or troops when required.
The consequences were far-reaching. Tribute flowing into the capital helped Tenochtitlan grow into one of the largest cities in the world at that time, with causeways, canals, markets and monumental temples. Yet the system rested on coercion, and many subject peoples resented it. Some provinces rebelled repeatedly, and some city-states, such as Tlaxcala, remained independent enemies. These tensions help explain why many indigenous groups allied with Hernán Cortés against Tenochtitlan in 1519-1521. The Triple Alliance fell with that conquest, but its tribute networks and city structures strongly shaped early colonial rule in Mexico.