Why was the Mexica system so vulnerable to human enemies?

The short answer

• Tribute and encirclement created enemies. The empire that fed Tenochtitlan rested on tribute from conquered peoples who kept their own rulers and resented the burden, and on the encirclement of independent rivals. Tlaxcala, surrounded by Mexica tributary provinces and reportedly cut off from salt and cotton, nursed long grievances, and Totonac lords complained of tribute collectors. When an outside force appeared, these peoples saw a chance to overturn Mexica rule, and many took it within months.
• Unfinished wars left strong rivals. The Alliance never conquered Tlaxcala, Huexotzinco or the Tarascan kingdom, whose army destroyed a Mexica force near Taximaroa around 1478–79, killing, by Durán’s probably inflated count, over 20,000 men. Fortified frontiers faced each other, with the Mexica fortress of Oztuma against Tarascan strongholds. These undefeated neighbours gave any invader potential allies and a secure base, which Tlaxcala provided for Cortés in 1520 when he retreated from the city with his army broken.
• A new disease met a population without immunity. Smallpox had never reached the Americas, so nobody had immunity, and it spread rapidly after arriving with Narváez’s expedition in 1520. The Nahua accounts in the Florentine Codex describe victims too covered with sores to move and many who starved because nobody could care for them. The epidemic killed the ruler Cuitlahuac after about eighty days in power and weakened the defenders before the siege, while sparing the Spaniards, most of whom had survived childhood exposure.
• The engineered lake had points of failure. Tenochtitlan depended on causeways for land access, an aqueduct for drinking water and canoes for food, all of which could be cut. The dike that had freshened the lagoon became an obstacle that Spanish brigantines, small sailing warships built on the lakeshore, broke through, and the aqueduct from Chapultepec was severed at the start of the siege. The very works that had made the city great gave a besieger clear targets, and the Spaniards’ allies knew exactly where they were.

What it changed

The fall of Tenochtitlan. The city fell on 13 August 1521 when Cuauhtemoc, the last ruler, was captured on the lake. Tens of thousands of people had died of hunger, thirst, disease and violence, and the chroniclers’ figures of up to 240,000 cannot be checked. A plaque set up 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 followed by further epidemics, including the cocoliztli of 1545, which struck during a severe drought recorded in tree rings. Central Mexico’s population fell by perhaps 80–90 percent within a century, from an uncertain starting point of perhaps 5–25 million to about 1 million. The labour that had maintained the lake system, its dikes, canals and fields, largely disappeared.

Spain inherits the lake. The Spaniards rebuilt the city on the ruins and inherited its hydraulic problems without the Mexica knowledge or labour that had managed them. Floods in the following century led them to begin draining the lake, a choice whose consequences, from the great flood of 1629 to the sinking of Mexico City today, the epilogue examines. The skills and labour that had kept the lake in balance were lost within a generation.