Why was the Mexica system so vulnerable to human enemies?

The short answer

• Tribute and encirclement created enemies. The tribute that fed Tenochtitlan came from conquered communities that retained their own rulers. Totonac lords complained about its collectors. Tlaxcala remained independent, but tributary provinces surrounded it and, according to complaints recorded by Spaniards, cut it off from salt and cotton. The arrival of an outside force gave subjects and rivals an opportunity to change their position. Many joined the Spaniards within months.
• Unfinished wars left strong rivals. Tlaxcala, Huexotzinco and the Tarascan kingdom had remained beyond Alliance control. Around 1478–79 a Tarascan army destroyed a Mexica force near Taximaroa; Durán's figure of more than 20,000 dead is probably inflated. Fortifications marked the frontier, including Mexica Oztuma and Tarascan strongholds. These undefeated states were potential allies for an invader. Tlaxcala proved especially consequential in 1520, when it sheltered Cortés's broken army and gave it a base for its return.
• A new disease met a population without immunity. Smallpox reached central Mexico with Narváez's expedition in 1520. The disease had not previously reached the Americas, and people without immunity faced its rapid spread. Nahua accounts in the Florentine Codex describe victims so covered with sores that they could not move and others starving because no one could care for them. Cuitlahuac died after about eighty days as ruler. The epidemic weakened the city's defenders before the siege, while sparing the Spaniards, most of whom had survived childhood exposure.
• The engineered lake had points of failure. An island city needed connections: causeways to land, an aqueduct for drinking water and canoes to carry food. A besieger could attack each one. The attackers severed the Chapultepec aqueduct at the start of the siege, while Spanish brigantines built on the lakeshore broke through the dike that had freshened the lagoon. Indigenous allies provided troops and knowledge of the targets. The works that made the city viable also gave its enemies places to interrupt its supply.

What it changed

The fall of Tenochtitlan. Tenochtitlan fell on 13 August 1521, when Cuauhtemoc, its last ruler, was captured on the lake. Hunger, thirst, disease and violence had killed tens of thousands; chroniclers' totals of up to 240,000 cannot be checked. A plaque erected at Tlatelolco in the 1960s reads, in translation, “It was neither triumph nor defeat; it was the painful birth of the mestizo people that is the Mexico of today.”

A population catastrophe. Smallpox in 1520 was only the beginning of a succession of epidemics. Cocoliztli struck in 1545 during a severe drought recorded in tree rings. Within a century central Mexico's population may have fallen by 80–90 percent, from an uncertain 5–25 million to about 1 million. Along with the devastation of families and communities came the loss of much of the labor that maintained the lake's dikes, canals and fields.

Spain inherits the lake. The Spaniards rebuilt on Tenochtitlan's ruins, but the basin's water problems remained. Much of the Mexica knowledge and labor used to manage them was lost within a generation. Floods in the following century led the newcomers to begin draining the lake. The epilogue follows that choice from the great flood of 1629 to the sinking of Mexico City today.