The expression Triple Alliance defines a military and political accord among the Aztec of Tenochtitlan, and other two Nahua cities of the Valley of Mexico: Texcoco and Tlacopan (Tacuba). This alliance ruled Central Mexico and most of Mesoamerica at the very end of the Postclassic period.
The Triple Alliance began in 1428, a date that traditionally marks the formation of the Aztec empire, after the combined forces of Tenochtitlan, Texcoco and Tlacopan defeated the Tepanecs of Azcapotzalco, until then the most powerful polity of the region. The three polities formed a military and economic alliance: they agreed not to engage in war against one another and to cooperate in wars of conquest against other towns.
The booty generated from these conquests was then unevenly divided as tribute among the three: two-fifths to Tenochtitlan, two-fifths to Texcoco and only one-fifth to Tlacopan. This tribute system guaranteed the three cities a flow of products coming from different environmental and cultural regions, increasing their power and prestige
Thanks to the strong alliance, the Aztecs managed first to consolidate their power within the Valley of Mexico and then to expand their dominion over most of Mesoamerica. Within the alliance, however, the Aztec/Mexica of Tenochtitlan soon assumed the prominent role, in term of political and military decisions, even if the formal tribute-sharing accord remained in place until the Spanish conquest.
Smith Michael, 2003, The Aztecs, Second Edition, Blackwell Publishing.
Van Tuerenhout Dirk R., 2005, The Aztecs. New Perspectives, ABC-CLIO Inc. Santa Barbara, CA; Denver, CO and Oxford, England.