‘Everything that has happened since the marvellous discovery of the Americas has been so extraordinary that the whole story remains quite incredible to anyone who has not experienced it at first hand. Indeed it seems to overshadow all the deeds of famous people of the past, no matter how heroic, and to silence all talk of other wonders of the world.’ – Bartolome de las Casas

It is amazing to think that when Bartolome de las Casas wrote those words in 1542, barely 20 years had passed since the discovery and conquest of the Aztec world in Mexico. It was only three years since the defeat of the Great Revolt of the Incas in the High Andes of Peru. At that moment, in fact, Manco Inca still controlled an independent Inca state in the jungles of Vilcabamba. During the same years in which Cortes overthrew the Aztecs, Magellan circumnavigated the globe.

For the first time, people discovered the true scale and shape of the earth. We are blasé about the pace of change in our own day, but has history, and our ways of seeing the world, ever moved so fast as it did in the 16th century? The conquest of much of the New World by Spanish conquistadors during those few years was surely one of history’s turning points. Indeed, as Karl Marx and Adam Smith claimed, perhaps it was the greatest event in history. There were many who thought so at the time.

‘When has it ever happened, either in ancient or modern times, that such amazing exploits have been achieved? Over so many climes, across so many seas, over such distances by land, to subdue the unseen and unknown? Whose deeds can be compared with those of Spain? Not even the ancient Greeks and Romans.’ – Francisco Xerez, Pizarro’s secretary, in his Report on the Discovery of Peru

The conquistador-turned-historian Pedro de Cieza de Leon agreed:

‘When I set out to write for the people of today and of the future, about the conquest and discovery that our Spaniards made here in Peru, I could not but reflect that I was dealing with the greatest matters one could possibly write about in all of creation as far as secular history goes. Where have men ever seen the things they have seen here? And to think that God should have permitted something so great to remain hidden from the world for so long in history, unknown to men, and then let it be found, discovered and won all in our own time!’ – Chronicle of Peru …

Out of this ferment of ideas came the first attempt in history to globalise justice and human rights. In the summer of 1550, in Valladolid, these great themes were aired before the King’s council. The Aristotelian scholar and humanist Juan Gines de Sepulveda argued for the civilising mission of Spain, so long as it was done humanely. The Indians were ‘natural slaves’ as Aristotle had defined the phrase, ‘inhumane barbarians who thought the greatest gift they could offer to God was human hearts’. People whose brilliant art and sculpture was no proof of their civilisation, ‘for do not even bees and spiders make works which no human can imitate?’

The great Dominican defender of Indian rights, Bartolome de Las Casas, brought a vast dossier of first-hand reportage to the hearing – as compelling an indictment of human cruelty as any modern report on the atrocities of Cambodia, Rwanda or Kosovo. His eloquent defence of the indigenous peoples ended with a noble cri de coeur: ‘All the world is human’. What is amazing is that the Spanish king actually listened. In a moment unique in the annals of imperialism, Charles V ordered the conquests to be stopped, while the issues were explored further.

However, as we know from our own time, ethical foreign policy will always run up against the cold reality of politics. Once the genie is let out of the bottle, history cannot be stopped. The Conquista continued. In a sense, it is still not over. Indeed what we are seeing now is a Second Conquista. For the global culture creeps with the electricity lines up even the loneliest valleys of the Andes. It will be as hard to resist as the first.

Today some modern scholars see the arguments outlined in the Valladolid debate as the forerunner of our own conception of human rights, and Las Casas as the first inspiration for the UN Declaration of 1948. A declaration in part prompted by the lessons of the past, and in part by the tragedies of contemporary history. Certainly the tragic dimensions of the 16th-century holocaust were apparent to people at the time. Many of the Spaniards were profoundly moved by what they had seen. The destruction of the last civilisations to have risen independently on the face of the earth, without contact with the world outside them.

Among these Spaniards were not only churchmen, like Sahagun, who fell in love with Nahuatl (Aztec) culture, but even the conquistadors themselves. Bernal Diaz, who marched with Cortes, was moved to compare the tragedy of Mexico with the Fall of Troy. On his deathbed, Mansio Serra de Leguizamon, one of the conquerors of Peru, expressed profound regret for the unjust destruction of Inca society: ‘I have to say this now for my conscience: for I am the last to die of the conquistadors.’

Read more about the conquistadors at bbc.co.uk