The siege of Tenochtitlán ended in August 1521 with the city starved, largely destroyed and its surviving inhabitants driven out; the campaign was fought by a few hundred Spaniards alongside tens of thousands of Tlaxcaltec and other indigenous allies, and a smallpox epidemic swept the city during the siege. The conquest state that followed rested on the encomienda, on tribute assessments inherited from the Mexica, and from the 1550s on congregación, the compulsory regrouping of dispersed settlements into towns to make labour and tithes easier to extract. Central Mexico then suffered the steepest recorded demographic collapse in the hemisphere, driven principally by successive epidemics — smallpox in 1520, measles in 1531, and the cocoliztli outbreaks of 1545 and 1576 that killed millions — compounded by forced labour, tribute demands, the disruption of agriculture and forced resettlement. The size of that collapse is disputed by a factor of five: the range of published estimates for 1519 runs from Rosenblat's 4.5 million to Cook and Borah's 25.2 million, and the choice of baseline determines whether the loss is counted in millions or tens of millions. McCaa notes that war mortality was of decidedly secondary importance beside epidemic disease, while stressing that overwork, ecological damage and forced relocation shaped who survived.
Cortés and his indigenous allies destroyed the Mexica capital and the Spanish crown replaced the Aztec tributary state with a colonial regime of encomienda, tribute and congregación resettlement
There is no census; every figure is a modern reconstruction from tribute lists, early colonial counts and carrying-capacity modelling, and scholars differ by a factor of five. The Berkeley school (Cook and Borah) put central Mexico at about 25 million; Sanders and Whitmore reconstruct 11-16 million; Zambardino argued for 5-10 million; Rosenblat's minimalist figure is 4.5 million. About 15 million sits in the middle of the credible range. Most reconstructions put the population at roughly 1-2 million by about 1600, though Rosenblat's minimalist series stays above three million.
The range simply reflects the disputed baseline. Robert McCaa's survey summarises Ángel Rosenblat's minimalist reconstruction as a fall from 4.5 million at contact to 3.4 million around 1650, a decline of 24 per cent, which implies a loss of about 1.1 million; Rudolph Zambardino's 5-10 million falling to about one million by the end of the sixteenth century implies four to nine million; and Sherburne Cook and Woodrow Borah's 25.2 million in 1519 falling to 1.2 million in 1620 implies about 24 million. Nearly all scholars attribute the great majority of these deaths to imported epidemic disease rather than to killing; forced labour, tribute exaction, famine and resettlement are treated as aggravating factors that raised mortality and suppressed recovery. Deaths in the fighting itself, including the siege of Tenochtitlán, are a small fraction of the total.
The starvation siege and destruction of Tenochtitlán, the massacre at the Templo Mayor festival in 1520 and at Cholula in 1519, compulsory labour drafts, and the destruction of temples and codices are described in the Spanish conquistador accounts, in the indigenous testimony collected in the Florentine Codex and in the crown's own inquiries.
Mexico