The Americas and Africa — Aztec and Mali as second regions

The big idea: The Aztec Empire ruled central Mexico from a capital that should not have been possible — a city of tens of thousands built in the middle of a lake.

By 1400 the Aztecs had turned a swampy island into one of the largest cities on Earth, feeding it through ingenious farming and running it on tribute taken from conquered neighbours.

Around 1325 the Mexica people — the group we call the Aztecs — settled on a marshy island in Lake Texcoco, in the Valley of Mexico. Legend said an eagle on a cactus marked the spot.

From that unpromising island grew Tenochtitlan, a capital that by 1400–1500 held perhaps 200,000 people — bigger than almost any city in Europe at the time.

Farming on water: the chinampa system: A lake island cannot easily feed itself, so the Aztecs invented the chinampa — a raised growing-bed woven from reeds, piled with mud and rooted to the lake bed by trees.

Thousands of chinampas turned the shallow lake into some of the most fertile farmland in the Americas, growing maize, beans and squash all year round.

A Spanish soldier's amazement: When Spanish conquistadors first saw Tenochtitlan in 1519, soldier Bernal Diaz wrote that its causeways and temples seemed like something from a dream — he had never imagined a city so large, so ordered, or so clean.

At the city's heart stood the Templo Mayor, a huge stepped pyramid with twin shrines to the war god Huitzilopochtli and the rain god Tlaloc.

Remember this: Tenochtitlan worked because the Aztecs solved a problem — how to feed a huge population on a tiny island — with engineering: chinampas for food, causeways for movement, aqueducts for water. Geography was a challenge they turned into a strength.

Tenochtitlan could not have grown so large on chinampas alone. Its true wealth came from an empire built through conquest — and kept loyal through fear of tribute collectors.

An empire of tribute, not direct rule: The Aztecs rarely governed conquered peoples directly. Instead they left local rulers in place but forced them to pay tribute — regular deliveries of maize, cotton cloth, cacao, gold, feathers and captives for sacrifice.

This meant a huge, sprawling area could be controlled by a relatively small ruling class in Tenochtitlan.

The pochteca: merchant-spies of the empire: Long-distance trade was run by the pochteca, a hereditary class of merchants who travelled in armed caravans far beyond Aztec borders.

They brought back luxury goods the empire could not tax as tribute — and quietly reported back on which towns looked weak enough to conquer next.

This was not a rigid caste system: a commoner who fought bravely in battle could rise in status, and captured warriors could be enslaved rather than instantly killed.

Contrast tribute with direct rule: A strong exam point is that the Aztec system relied on fear and distance, not permanent garrisons. Because subject peoples were left mostly self-governing, resentment simmered — which is exactly why so many joined Hernan Cortes against the Aztecs in 1519–21.

The big idea: While the Aztecs built an empire on tribute, the Mali Empire in West Africa built one on trade — controlling the routes that carried gold north and salt south across the Sahara.

Mali's wealth became so famous that one ruler, Mansa Musa, is still remembered as possibly the richest person in history.

From around 1235, Mali grew into a vast West African empire stretching along the Niger River, built on farming, herding and — above all — control of the trans-Saharan trade.

The gold-salt trade: West Africa held rich goldfields but almost no salt, which was vital for preserving food and health in a hot climate. North Africa had huge salt deposits but craved gold.

Mali's rulers taxed the camel caravans that carried gold north and salt south, growing fabulously wealthy simply by sitting on the route between the two.

Mansa Musa's hajj, 1324: Mali's most famous ruler, Mansa Musa, travelled to Mecca in 1324 on hajj with a caravan said to include thousands of soldiers, servants and camels loaded with gold.

He reportedly gave away so much gold passing through Cairo that its value crashed in Egyptian markets for years afterward — a story that put Mali on European maps for the first time.

Mansa Musa ruled as a devout Muslim, and Islam had spread into West Africa through these same trade contacts, especially among rulers and merchants.

Timbuktu: a city of scholars: The trading city of Timbuktu grew into a major centre of Islamic learning, with mosques, libraries and universities such as Sankore.

Scholars travelled from as far as Cairo and Mecca to study and copy manuscripts there — proof that Mali's wealth bought intellectual prestige as well as gold.