How did three cities rule an empire by collecting tribute?

Three cities ruled the Aztec empire together, and they did it mostly by collecting tribute rather than by governing every town directly. The empire was an alliance of Tenochtitlan, Texcoco and Tlacopan, which ruled in and around the Valley of Mexico from 1428 until the Spanish and their native allies defeated them in 1521.

The alliance began as a war. Tenochtitlan, Texcoco and Tlacopan, with a fourth ally, defeated the Tepanec city of Azcapotzalco in 1428, and in 1430 the three cities formed a treaty. Their leaders agreed to cooperate in future wars of conquest, and the tribute was split so that Tenochtitlan and Texcoco took two shares each and Tlacopan one.

The rule it set up was surprisingly light-handed. The Aztecs left the rulers of conquered cities in power, so long as they paid tribute twice a year and sent soldiers when needed. In return, the imperial authority offered protection and political stability and an integrated economic network, while the conquered kept significant local autonomy. It was more a system of tributes than a single unitary government.

The weakness of that bargain was that rebellions were commonplace, because rule was indirect. Over time Tenochtitlan became the largest, most powerful and most influential of the three cities, the de facto center of empire. And the same resentment against the tribute collectors would later be useful to a Spanish captain looking for allies.