Was the lake an advantage or a burden for the Mexica?

The short answer

• A closed basin made salt and floods inevitable. Water draining from the mountains had no river outlet from the basin. Evaporation removed water and left salts, especially in low-lying Texcoco; in wet years the level of the connected lakes rose. Settlers could use the lakes, but they also had to contend with salt and floods. These conditions would shape Mexica engineering for a century.
• Latecomers took the marginal land. Established cities such as Azcapotzalco, Culhuacan and Texcoco already held the best farmland on the shores when the Mexica arrived in the thirteenth century. After expulsion from Chapultepec and Culhuacan, the newcomers found refuge on islands in the western marshes that others had not claimed. Their later power grew out of this weakness, helping explain the value they placed on military service and organised labour.
• Service to Azcapotzalco taught war and politics. Azcapotzalco’s authority imposed tribute and military service on the Mexica for about a century. Campaigns across the basin gave them experience, while marriages connected them to powerful families. They also saw how a dominant city drew tribute from others. When Tezozomoc died in 1426 and his Tepanec kingdom split, that experience could be turned against their overlords.
• An alliance of the threatened. Maxtla’s enemies had reasons to set rivalry aside. He threatened the Mexica and Nezahualcoyotl, whose father the Tepanecs had killed in 1418; some Tepanec towns, including Tlacopan, also opposed him. A common danger brought them together. Like later alliances formed against a shared enemy, their coalition would become an instrument of expansion.

What it changed

A Triple Alliance to share tribute. The partners campaigned together and, according to later chronicles, divided tribute in shares: two-fifths each for Tenochtitlan and Texcoco, one-fifth for Tlacopan. Within a few years they subdued Xochimilco, Cuitlahuac and other southern lake towns. The alliance gained both the basin’s best chinampa land and the labour of those who farmed it.

Engineering the lake becomes possible. Rulers who could call workers from towns around the lake could attempt larger defences and water supplies. Chapter 8 follows two such works: the masonry aqueduct from Chapultepec and Nezahualcoyotl’s great dike of about 1449. Their labour depended on the political power gained in 1428.

Expansion beyond the basin. Azcapotzalco’s defeat gave the alliance access to tribute networks beyond the basin. Itzcoatl and his successor Moctezuma I then sent campaigns south into Morelos and north into Otomí lands. The growing empire would bring maize, beans and cloth to Tenochtitlan from hundreds of kilometres away, while creating enemies as well.