The big idea: Between 1519 and 1533, small bands of Spanish adventurers called conquistadors destroyed two of the largest empires on Earth — the Aztec and the Inca.
This was not a fair fight. Spanish steel, horses and, above all, disease combined with local political splits to bring down empires of millions in a matter of months.
In 1519 Hernán Cortés landed on the coast of Mexico with roughly 500 men. He marched inland to Tenochtitlan, the Aztec capital, a city of perhaps 200,000 people — far bigger than any city in Spain.
Cortés did not fight the Aztec Empire alone. He allied with the Tlaxcalans and other peoples who deeply resented Aztec rule and its demands for tribute and sacrifice victims. Without these local allies, conquest would have been impossible.
The Aztec emperor Moctezuma II hesitated, and in 1520 the Spanish were driven out of the city in a bloody retreat. But Cortés returned in 1521 with his allies and, helped by a devastating smallpox epidemic already sweeping the city, captured Tenochtitlan and ended the empire.
- Hernán Cortés — led the small Spanish force that brought down the Aztec Empire between 1519 and 1521
- Tenochtitlan — the Aztec capital, captured in 1521 and rebuilt as Mexico City
- Tlaxcalans — indigenous allies whose resentment of Aztec rule was decisive to Cortés's victory
- Francisco Pizarro — led a smaller force that toppled the Inca Empire in Peru by 1533
Peru: an empire already in crisis: In 1532 Francisco Pizarro landed in Peru with barely 180 men. He found the huge Inca Empire weakened by a civil war between two rival brothers, Atahualpa and Huáscar, fighting over the throne.
Pizarro captured Atahualpa by trickery at Cajamarca, extracted a legendary ransom of gold and silver, then executed him anyway in 1533 — clearing the way for Spanish control of the whole empire.
Why so few men won so much: Never explain the conquest with weapons alone. Political division (Tlaxcalan resentment, the Inca civil war), disease already weakening populations before battles were even fought, and local allies doing much of the actual fighting all mattered as much as steel swords and horses.
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Conquest was only the start. Spain now had to organise millions of indigenous people and vast new territories into a working colonial economy — and it did this through a brutal system of forced labour.
The encomienda system: Under the encomienda, the Spanish Crown granted a colonist control over a group of indigenous people.
In theory, the colonist protected them and taught them Christianity. In practice, it was forced labour in mines and fields, often worked to death, with almost no real protection at all.
- Encomendero — the Spanish holder of an encomienda grant, who profited from the labour and tribute of the people assigned to him
- Potosí — a mountain of silver in modern Bolivia, worked by forced indigenous labour under a system called the mita
- Tribute — goods, crops or labour indigenous communities were forced to hand over to their Spanish overlords
- Repartimiento — a later, supposedly milder labour draft that replaced encomienda in some areas but still forced unpaid or underpaid work
Demographic collapse: The single greatest disaster of conquest was disease. Indigenous peoples had no immunity to smallpox, measles and influenza carried by the Spanish.
Historians estimate the population of central Mexico fell from around 20-25 million before 1519 to under 2 million a century later — a collapse of over 90 percent. This is known as the demographic collapse, and it happened across the whole Americas, not just Mexico.
Disease arrives first
Smallpox often spread ahead of the Spanish themselves, passed along trade and travel routes, so many communities were already devastated before ever meeting a conquistador.
Forced labour adds to the toll
Survivors were then driven into mines like Potosí and onto plantations under encomienda and mita, with brutal conditions that killed still more.
Societies are shattered
Whole communities lost their farmers, priests and leaders, breaking down traditional structures of government, religion and family life.
Disease struck first, forced labour finished the job, and indigenous society was left in ruins.
Why this matters for essays: Demographic collapse was not a side-effect of conquest — it was the single biggest force reshaping the Americas. It explains why the Spanish soon began importing enslaved Africans: with the indigenous workforce dying by the millions, colonists needed new forced labour to replace it.
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Conquest also brought a religious mission. The Spanish Crown and the Catholic Church saw converting indigenous peoples to Christianity as a duty — and a justification for empire.
Conversion and syncretism: Missionary orders such as the Franciscans and Dominicans baptised millions and built churches across the new colonies, often on the very sites of destroyed temples.
But conversion was rarely total. Indigenous peoples frequently blended old beliefs with the new faith, a process called syncretism. A famous example is the reported 1531 appearance of the Virgin of Guadalupe to an indigenous man, Juan Diego — a devotion that fused Catholic and native imagery into something distinctly Mexican.
The Columbian Exchange: Contact between the Americas and the rest of the world triggered the Columbian Exchange.
Maize, potatoes and tomatoes crossed the Atlantic and reshaped diets in Europe, Africa and Asia. Horses, wheat and cattle came the other way — but so did the diseases that caused the demographic collapse. Silver from Potosí flooded world trade and helped fund Spain's empire.
Went from the Americas
- Maize, potatoes, tomatoes, cacao
- Silver and gold from mines like Potosí
- Tobacco
- New knowledge of unfamiliar lands
Went to the Americas
- Horses, cattle, pigs and wheat
- Sugar cane, worked by forced labour
- Smallpox, measles and influenza
- Christianity and European law and language
The las Casas debate: Not every Spaniard accepted the brutal treatment of indigenous peoples. The Dominican friar Bartolomé de las Casas, once an encomendero himself, became the fiercest critic of the system.
In his 1552 account, he documented mass killings and forced labour, arguing indigenous peoples were fully rational humans with natural rights, not lesser beings to be exploited.
The Valladolid debate, 1550-1551: Las Casas argued in person against Juan Ginés de Sepúlveda, who claimed indigenous peoples were natural slaves who benefited from Spanish rule and conversion.
No side won outright, but the debate helped push the Crown to pass the New Laws (1542), which limited (though never fully ended) encomienda abuses. It shows that the treatment of indigenous peoples was a genuine, live controversy inside Spain itself — not something everyone simply accepted.