The first conquerors
The Inca empire was based on the aggregated technological knowledge and expertise of three millennia of cultures in the region. Inca religion, architectural and engineering abilities, accounting and organisational skills, crafts and arts were all inherited from previous societies that had ended equally suddenly - for example, the Mochica civilisation, which never recovered after a change in the El Nino weather cycle in about AD 600. So although the scale of Inca conquest, public works and social organisation was awesome, the empire was less than a century old by the time it fell.

And it was the creation of just two ambitious men. The real founder was Pachakuti, meaning 'cataclysm', who with his son Tupac Yupanqui came out of the Inca homebase in the Cuzco valley around 1440, at a time of intertribal threat. Father and son were charismatic military commanders, and during their succeeding reigns over the next 50 years, they conquered ceaselessly to create the largest-ever American empire, expanding through Ecuador, Argentina, Chile and into Bolivia.

The momentum of conquest (and of the clever imperial family diplomacy that offered peaceful inclusion in the empire as an alternative to conquest) depended on those two men. The structured, public nature of Inca society was given meaning and purpose by the conquests - as was British society during the height of Victorian imperialism.

The last emperor
However, the next to succeed, Huayna Capac, who became Supreme Inca in 1493, did not have the genius of his father and grandfather. He lost a little, gained nothing and was worn away by suppressing with extreme cruelty the revolts of the subjugated (including the attempted invasion by an Argentinian grouping, the Chiriguano), and by repairing damage caused by a serious earthquake. He was also worried by ominous signs and by rumours from the Caribbean and Central America, where Europeans had landed, determined on conquest, and brought diseases against which the native Americans had no immunological defence.

Huayna Capac died in 1527 of smallpox, which seems to have been transmitted from Spaniard to Aztec to Guatemalan Mayan and coastal Ecuadorian contacts before ravaging the Inca port of Tumbes. He died so swiftly that he had no time to choose a successor.

The two contenders
The Supreme Inca was an autocrat, but there was also an aristocracy and a court. Each Supreme Inca was the centre of a clan grouping; when he died, his income from the lands he had conquered went to his male clan descendants. The new emperor had to conquer new lands to establish fresh personal wealth.

There were two contenders for the throne: Huayna's official son Huascar, endorsed by the nobility and priesthood in Cuzco; and one of Huayna's concubine-bred regional sons, Atahuallpa, who had command of most of the army and its leaders. At first, Atahuallpa claimed the governorship of his home region of Quito; then he said that his father had split the empire into two, and he was lord of the north; then he campaigned against Huascar for total domination.

Enter the Spaniards
Five years later, on 15 November 1532, Francisco Pizarro and his 175 Spanish conquistadores and 120 horses reached the highland town of Cajamarca just as the Atahuallpa-Huascar civil war was ending with the defeat and capture of Huascar and the massacre of his immediate family, wider clan and clan-labourers. Pizarro, who had spent a year travelling from his landfall in Ecuador, met Atahuallpa at the hot springs outside the town and, at the Supreme Inca's invitation, advanced inside the gates.

Atahuallpa may have intended to use the Spaniards - and their amazing new horses (which he thought were useless at night) and their arquebus weapons that made the sound of thunder - to establish himself unshakeably as emperor in Cuzco and rebuild the empire. He told his men to treat the conquistadores respectfully and courteously, describing them as 'messengers from God'. This may have been flattery, or an allusion to the stories of the god Wiraqocha, associated with the end of the Inca empire.

The Spaniards crucially decided that killing or capturing the autocratic Supreme Inca was their one chance of survival, let alone conquest. Dissidents against his reign, who had helped them thus far, would no longer be of any use.

Blitzkrieg
The following day, Pizarro's men hid themselves in the buildings of Cajamarca, and placed their cannon strategically on a platform in the centre of town. Atahuallpa and 6,000 unarmed men arrived in the town square in an impressive formal procession. The Supreme Inca was carried on a litter, surrounded not by a bodyguard but by his senior counsellors and imperial officials - the most important organisation men in his empire.

The Spaniards' priest and his incompetent interpreter read the Incas the Conquistadore, the Catholic equivalent of the Riot Act. Atahuallpa demanded the return of state property that the Spaniards had stolen from storehouses en route, and tossed away the Bible he was proffered. The priest, in a yell, gave Pizarro religious sanction to attack; trumpets sounded and the cannon was fired. The armoured Spanish, with Toledo swords, charged the Incas, causing panic. In that panic, Atahuallpa's bearers were killed, his litter overturned and he was taken prisoner, and most of his procession was butchered in the crush.

The sword-arm reach of the Spaniards gave them a weapon advantage to follow up the profound psychological shock of their attack on the sacred person of Atahuallpa, who never had the chance to order his men to defend themselves. It was not a set-piece battle but a blitzkrieg coup, all bold plan, stun and death.

A ransom of gold and silver
The few Incas who, when a wall gave way, had escaped from the pen of the square were hunted down by Spaniards on horseback. The 25,000 Inca troops outside the city - who had lost their god-emperor and all their senior commanders in under an hour - did not, with Atahuallpa as hostage, have the organisation or authority to strike back.

That night, Atahuallpa seems to have lost his sense of himself as Supreme Inca and of the meaning of his empire. He offered Pizarro a ransom, but to the Inca, gold had never signified money or wealth, but was a rare substance for artworks with a profound religious and social meaning - the sweat of the Sun. Atahuallpa offered to fill his prison room with gold, and twice more with silver, within two months. That promise proclaimed the Inca empire already at an end. Pizarro later had Atahuallpa strangled.

A question of timing
Although the crucial event in the downfall of the Inca empire occurred on that late Saturday afternoon of 16 November 1532, no single reason explains the successful Spanish conquest of Peru by such a tiny invading force.

Most of the evidence we have about pre-conquest Inca society, and its overthrow is based on the writings of contemporary Spaniards or later chroniclers who were Spanish or of mixed Spanish-Inca heritage. This is like trying to understand the fall of Communism in Eastern Europe or the disintegration of the Soviet Union through the writings of hardline Stalinists - they keep missing both the signs and the point. It may simply come down to a question of timing. At just that moment in that year, the Empire of the Sun was at its lowest ebb and so extremely vulnerable to Pizarro's small but lucky army.