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

The short answer

• A closed basin made salt and floods inevitable. Because the Basin of Mexico had no river leading out of it, every drop of rain that fell on the surrounding mountains ended in the lakes and left only by evaporation. That simple fact made Lake Texcoco salty and meant that wet years raised the water level across the whole system. Any society that settled on the lake inherited these two problems, which would shape Mexica engineering for the next century.
• Latecomers took the marginal land. When the Mexica arrived in the basin in the thirteenth century, the best farmland around the shores already belonged to established city-states such as Azcapotzalco, Culhuacan and Texcoco. Driven from Chapultepec and then from Culhuacan, they took refuge on islands in the western marshes that nobody else wanted. Their later empire grew from a position of weakness, which helps explain why they prized military service and organised labour so highly.
• Service to Azcapotzalco taught war and politics. For about a century the Mexica fought as mercenaries for the Tepanec kingdom of Azcapotzalco and paid it tribute, campaigning for its rulers across the basin. This service gave them military experience, marriage ties with powerful families and a model of how a dominant city could collect tribute from others. When the Tepanec kingdom split after its ruler Tezozomoc died in 1426, the Mexica were ready to turn those lessons against their overlords.
• An alliance of the threatened. Maxtla, the new ruler of Azcapotzalco, threatened both the Mexica and Nezahualcoyotl, whose father had been killed by the Tepanecs in 1418, and some Tepanec towns such as Tlacopan opposed him too. Their shared danger made cooperation possible among cities that had often been rivals. The Triple Alliance grew from a coalition of the threatened, much as modern alliances often form against a common enemy before they become instruments of expansion.

What it changed

A Triple Alliance to share tribute. Tenochtitlan, Texcoco and Tlacopan agreed to campaign together and to divide tribute, with the two larger partners taking two-fifths each and Tlacopan one-fifth, according to the later chronicles. Within a few years the alliance had subdued the southern lake towns of Xochimilco and Cuitlahuac, giving it control of the best chinampa land in the basin and the labour of the towns that farmed it.

Engineering the lake becomes possible. With the shores in friendly hands, the alliance could mobilise labour from many towns to rebuild the city’s water supply and protect it from floods. Chapter 8 follows Nezahualcoyotl’s great dike of about 1449 and the masonry aqueduct from Chapultepec, both of which depended on the political power won in 1428 to call up workers from towns all round the lake.

Expansion beyond the basin. By defeating Azcapotzalco, the alliance also inherited its tribute networks outside the basin. Under Itzcoatl and his successor Moctezuma I the alliance began campaigning south into Morelos and north into the Otomí lands, building the tribute empire that would later feed Tenochtitlan with maize, beans and cloth from hundreds of kilometres away, and that would one day make the alliance many enemies.