The Aztec Triple Alliance of Tenochtitlan, Texcoco, and Tlacopan formed in 1428.
The alliance emerged during the Tepanec War, when Tenochtitlan and Texcoco joined forces against Azcapotzalco, the dominant power in the Valley of Mexico. Tlacopan, a Tepanec city, joined the victorious coalition. Together, the three Nahua city-states created the political and military structure later called the Aztec Empire.
The alliance was not a centralized nation in the modern sense. Each member retained its own ruler and government, while the partners coordinated warfare, divided tribute, and shared authority over conquered territories. Tenochtitlan gradually became the most powerful member, although Texcoco remained an important center of law, scholarship, and administration.
Some accounts place the formal treaty in 1430, which explains occasional variation in reference works. Nevertheless, 1428 is the standard date for the alliance’s formation and the beginning of its imperial expansion. The coalition lasted until the Spanish conquest of Tenochtitlan in 1521.