Rose from a great impure
Sometime around 1100 CE the city-states or altepetl which were spread over central Mexico began to compete with each other for local resources and regional dominance. Each state had its own ruler or tlatoani who led a council of nobles but these small urban centres surrounded by farmland soon sought to expand their wealth and influence so that by c. 1400 CE several small empires had formed in the Valley of Mexico. Dominant amongst these were Texcoco, capital of the Acholhua region, and Azcapotzalco, capital of the Tepenec. These two empires came face to face in 1428 CE with the Tepanec War. The Azcapotzalco forces were defeated by an alliance of Texcoco, Tenochtitlan (the capital of the Mexica) and several other smaller cities. Following victory a Triple Alliance was formed between Texcoco, Tenochtitlan and a rebel Tepanec city, Tlacopan. A campaign of territorial expansion began where the spoils of war - usually in the form of tributes from the conquered - were shared between these three great cities. Over time Tenochtitlan came to dominate the Alliance, its ruler became the supreme ruer - the huey tlatoque ('high king') - and the city established itself as the capital of the Aztec empire.
The empire was kept together through the appointment of officials from the Aztec heartland, inter-marriages, gift-giving, invitations to important ceremonies, the building of monuments and artworks which promoted Aztec imperial ideology, and most importantly of all, the ever-present threat of military intervention. Some states were integrated more than others whilst those on the extremities of the empire became useful buffer zones against more hostile neighbours, notably the Tarascan civilization.