Cusco, in the high valleys of the Andes, was one of many small chiefdoms when, in the early 15th century, it was attacked by the neighbouring Chanca confederation. The traditional story says that the elderly Inca Viracocha fled the city, but his son Cusi Yupanqui stood, defeated the Chanca, and ascended the throne in 1438 as 'Pachacuti' ('he who shakes the earth'). The date is more symbolic than precise — the real pace of Inca expansion may have been slightly earlier or later — but current archaeology broadly agrees on a marked state-building surge in the mid-15th century.

Pachacuti and his successors Topa Inca Yupanqui and Huayna Capac brought, within about ninety years, a region stretching from northern Ecuador to central Chile, from the Andean coast to the edge of the Amazon, under a single administration. Tahuantinsuyu, the 'Four Regions', was divided into four administrative quarters; the Qhapaq Ñan road network linked them to Cusco with a total length of some 40,000 kilometres, organising traffic through mountain passes, suspension bridges, and waystations (tambo). Relays of runners (chasqui) carried messages along these roads; fresh fish from the Pacific was said to reach Cusco within days.

The Incas had no writing, but the knotted, coloured cord records — quipu — tracked tax amounts, censuses, storehouse inventories, and possibly narrative data. Quipu has not been fully deciphered; some served accounting, some likely as a memory aid for oral history. The state built its great infrastructure through a rotating community-level labour obligation (mit'a), in a system where land was held by community rather than person: terraced fields, water channels, storage cities, and engineering feats like the massive polygonal stone walls of Sacsayhuamán above Cusco. Machu Picchu was founded around 1450 in the reign of Pachacuti as a royal estate and ritual complex.

The Inca state would unravel after about 1532 with the arrival of Francisco Pizarro's small Spanish force, but that collapse owed much to internal civil war (between Huáscar and Atahualpa), the smallpox epidemic from Europe (which probably killed Huayna Capac), and the initial shock of cavalry and firearms. Inca infrastructure — roads, terraces, stone walls — nevertheless survives and is still in use; Quechua is still spoken widely across the Andes. It is one of the clearest medieval examples that a civilization without writing is neither 'stateless' nor 'primitive'.