Giles Tremlett is a Fellow of the Cañada Blanch Centre at the London School of Economics and Contributing Editor to the Guardian. He was Madrid correspondent for the Economist until 2016. He was also Madrid correspondent for the Guardian for 12 years. He has lived in, and written about, Spain for the past twenty years, and is the author of Catherine of Aragon: The Spanish Queen of Henry VIII and Ghosts of Spain: Travels Through Spain and Its Silent Past.

No woman in history has exceeded her achievement,’ Hugh Thomas.

The dazzlingly dressed 23-year-old woman who processed down the chilly streets of the Spanish city of Segovia in order to claim the crown of Castile in December, 1474, was peculiar in many ways. Isabella of Castile was no black-haired, dark-eyed, Spanish beauty. She had, rather, soft green-blue eyes and the kind of pale auburn hair that, even today, is categorized by Spaniards as “blonde”. She owed her colouring and, perhaps, some of her steely ambition to an English grandmother – John of Gaunt’s strappingly-built daughter, Catherine of Lancaster, who had married a king of Castile and acted as regent for her son. Those who lined the narrow streets of this wind-swept central Spanish city were impressed by Isabella’s glittering jewellery and rich clothes, but were even more amazed by this young woman’s boldness. For Isabella was claiming the right to rule most of modern-day Spain – a move that, when her husband Ferdinand inherited the neighbouring crown of Aragon, would give the young couple control of almost the entire territory.

Ahead of Isabella walked one of her gentlemen, the robust and ruddy-faced Gutierre de Cárdenas. He held the royal sword in front of him, with its point towards the ground. This weapon was a symbol of royal might as potent as any crown or sceptre. If Isabella’s jewels spoke of regal magnificence, Cárdenas's sword threatened violence. Both indicated power and a willingness to exercise it. “Some of those in the crowd muttered that they had never seen such a thing,” reported one contemporary. The grumblers felt free to question the right of a woman to rule over them, and little need to keep their mouths shut in public. The monarchy had long been weak and female sovereigns were a distant and dismal memory in Castile. European history, meanwhile, offered only a handful of examples of successful queens. With so few models to follow as queen regnant, Isabella would have to make it up herself.

For most of the century Castile’s kings had been playthings of the mighty, aristocratic magnates who called themselves 'Grandees'. Only a modest number of the latter – or of bishops and other senior officials - accompanied Isabella as she had herself proclaimed queen on a hastily erected stage in the Segovia’s main square. It was a sign that her problems went beyond her sex and the fragile state of Castile's monarchy. For Isabella was not the only claimant to the throne, nor was she the person who had been designated as such by the previous monarch. This was, in short, a usurper's pre-emptive coup.

The prize she claimed was, despite the years of poor rule, considerable. Castile was the largest, strongest and most populous kingdom in the Iberian peninsula – stretching from the wet and stormy Atlantic coast in there north to the sun-drenched beaches of the south. With upwards of four million inhabitants it had significantly more people than England and was one of the larger countries in Europe. Castile was the result of a slow, six-centuries-long conquest of land that had been occupied by the Muslims – known to Christians as moros, or Moors - when they swept through Iberia early in the eighth century. The Moors still occupied part of Spain, with the Nasrid dynasty ruling most of the southern, Mediterranean coast from the luxurious Alhambra palace complex in Granada.

Isabella appeared after more than a century of crisis in Europe, with disease and constant Moslem encroachment stoking millennial fantasies of a godlike monarch who would revive ailing Christendom’s ailing fortunes. In 1346 the Black Death had reached western Europe, where it killed a third of the population. Then, in 1453, the dashing young Ottoman sultan Mehmet II ordered that his galleys be pulled overland into the Golden Horn, cutting off the capital of eastern Christendom, Constantinople. This soon fell into his hands. Moslem armies then conquered Greece and much of the Balkans, marking a new low point in the history of western, Christian Europe.

Castilians had hoped that the great saviour of Christianity might be one of their own monarchs, but weak kings brought constant disappointment. Foreigners saw squabbling Spain as shrouded in ‘natural darkness’, and Castile remained a volatile, unsettled society. A whole new social category, the ‘new Christians’ or conversos, was still being assimilated amid frequent outbursts of violence. The conversos were the children and grandchildren of what had once been the world's largest community of Jews, most of whom were forcibly converted eighty years earlier. In the cities a growing bourgeoisie of wool traders, bankers, merchants and local oligarchs struggled to assert itself. But real power lay in the vast, untaxed estates of Grandees, military orders and the church – who used their muscle to bully the country’s monarchs. Yet in a continent split into dozens of bickering territories, Castile was one of the few countries with the potential to produce a leader who could reverse the flagging fortunes of western Christendom. Vast flocks of hardy, fine-haired merino sheep – some five million valuable animals – had turned it into what one historian has called ‘the Australia of the Middle Ages’, with wool travelling north to the sophisticated textile centres of Europe. In Rome, the spiritual capital of Europe, the pope was only too aware of the importance of this wealth since Iberia provided a third of the papacy's income.

Isabella’s claim to the throne was weak, but not groundless. Her predecessor, and half-brother, Henry IV had proclaimed her heiress six years earlier. But, after the wilful 17-year-old disobeyed him by marrying Ferdinand, he had changed his mind. Henry had then named his own daughter, Juana “la Beltraneja”, as heiress instead. Many believed that Juana was not his real daughter, since he suffered from a debilitating medical condition called acromalgy and was thought to be impotent. Court gossips whispered that his daughter’s biological father was really one of Henry’s senior officials. That was sufficient for Isabella, who claimed that this supposed but unproven illegitimacy barred Juana from ruling. When neighbouring Portugal sent an army to back her rival, Isabella turned to God – who, in her mind, ultimately decided battlefield victories - to be her judge. And when she and Ferdinand won that war, she saw it as final proof that she was divinely elected. That belief - that she was God’s tool - reinforced an already unshakeable sense of self-confidence. Over the next 30 years she would govern, in remarkable partnership of equals with Ferdinand, with that in mind. Power became her passion, and her vice. ‘This queen of Spain, called Isabella, has had no equal on this earth for 500 years,’ one awestruck visitor from northern Europe would eventually proclaim. It was not hyperbole.

Although Isabella imposed power and order slowly over time, it is the year of 1492 that has come to symbolize the dramatic importance of Isabella’s reign to world history. The year could not have started better. On 2nd January, a Christian flag bearing the Cross of St James was raised above the Alhambra palace complex, signifying the final surrender of Moslem Granada. Isabella waited outside the city as 500 Christian prisoners, still in chains, were released and the last Nasrid king, Boabdil, rode off through the melodramatically named Pass of the Moors Last Sigh. The conquest of Granada brought an end to more than six centuries of Moslem rule on the Iberian peninsula and placed the last major piece into the jigsaw puzzle that became Spain – with Spaniards now seeing Isabella’s reign as the moment their nation was born. It also saw embattled Christendom regain land off the Moslems for the first time in many decades. Celebrations were held across the continent, with Henry VII and his court attending a lavish thanksgiving service at St Pauls in London.

Granada’s inhabitants were initially treated much like the historic Moslem communities that had continued to exist across Christian Castile – where the right to worship had been the central pillar of a remarkable period of religious tolerance that had seen Jews, Moslems and Christians live side-by-side for centuries. Three months after their surrender, however, Isabella and Ferdinand brought that tradition to a dramatic end. They signed a decree announcing the expulsion from Castile of what was then the world’s largest Jewish community, forcing tens of thousands of refugees into boats or across the border into Portugal. Their experiences of hunger, homelessness, violence, rape and murder now form part of the long Jewish story of exile and forced displacement. It is no coincidence that the first expulsion order was written by the head of the Spanish Inquisition, Tomas de Torquemada. He was one of a handful of pious, sometimes radical, friars to whom Isabella turned for advice. Torquemada had long been urging her to carry out a campaign of religious cleansing. The Spanish Inquisition itself, set up at Isabella and Ferdinand’s request – and with them in charge of appointing officials and carrying out punishments like burning at the stake – was an example of that. This targeted only the conversos in what became the systematic terrorising of a minority whose main crime was their Jewish bloodline.

Isabella’s iron grip on her kingdom, however, also permitted her to embark on adventures of long-term, global importance. For years a colourful Genoese sailor called Christopher Columbus had been pestering her for support. He wanted to sail west across the Atlantic Ocean in order to reach China, Japan and India. No-one knew how wide this ocean was, or if anything lay between Europe and Asia, so Isabella turned Columbus down several times. But in 1492, with the wind of fortune behind here, she changed her mind. It required a relatively small amount of money to send him off – and Isabella probably thought he would disappear in the middle of the vast sea. But when Columbus returned with hammocks, colourful parrots, canoes and indigenous tainos from what is today Santo Domingo, (having discovered the islands of the unknown continent of America, rather than Asia), the event marked the beginning of a tectonic shift in global power. Isabella was now queen of lands on the other side of the Atlantic and embattled Christendom - a relatively weak entity when compared to China or the great eastern powers - was able expand into vast lands across the Ocean. Atlantic nations like Spain, England, Holland and Portugal drove a first wave of globalisation by sea that would eventually give birth to the great western empires and superpowers - of Spain, Great Britain and the United States. The triumphant march of “western civilisation”, in other words, started in Isabella’s Castile.

By the start of the 16th century, Isabella’s Spain was recognized as the new major power-player in Europe – the only real rival to France. Within a century it would become the continent’s dominant nation, with an empire on which the sun never set and great fleets carrying American silver into its exchequer.

Historians have puzzled over the secret to the success of this first great European queen – a woman who provided a model for her granddaughter, England’s Queen Mary I, and for other great queens like Elizabeth I, Victoria or Catherine the Great. Some point to her remarkable marriage and a husband, Ferdinand, who either could not, or did not want to, control his wife. They proved a dynamic, trusting and effective partnership – with Isabella happily jumping on horses and riding for days to put out fires across her kingdom, while Ferdinand saddled a horse and rode off in a different direction to do the same. In public, Isabella maintained a dignified, regal air of distant majesty that clearly terrified the men around her. Trusted advisors noted that it was hard to change her mind and, while some found her too harsh in her treatment of conversos and ordinary criminals, her subjects found themselves living in an unusually harmonious and prosperous nation. She never, however, felt a need to feign any form of masculinity. Whereas Elizabeth I would later proclaim herself to have ‘the heart and stomach of a king’, Isabella preferred to express angry astonishment that ‘as a weak woman’ she was so much more audacious and belligerent than the men who served her. Confused men could not work out how to assess a woman who was so clearly dominant. They either praised her for possessing “manly qualities” or turned to the poetry of courtly romance - which allowed them to express both adoration and fear of a woman – to explain their feelings towards her.

As the mother of four daughters, she was a pioneer in women’s education. Just as she, late in life, rued never having been taught Latin (and studied it with the help of a female tutor, Beatriz Galindo), so she insisted that her daughters had the best education money could buy. A contemporary reported that her second daughter (and eventual heiress) Juana “answered instantly in Latin to any question she was asked, just like the princes who travel from one land to another. The English say the same thing of Catherine [of Aragon], Juana's sister. And the whole world has similar praise for the [other] two sisters.” Even the arrogant Dutch humanist Erasmus was impressed, finding Catherine to be “well instructed – not merely in comparison with her own sex… And is no less to be respected for her piety than her erudition.” But, despite this, Isabella did nothing to promote women as a whole. She was, rather, a traditionalist who thought they should maintain a secondary role to their husbands. And her daughters were kept under close vigilance, with constant warnings to avoid men and “the madness of love”.

Isabella might have gloried in all her many political triumphs, but a never-ending saga of family tragedies made that difficult. In a short period of time she lost three of her favourites – her son Juan, her daughter Isabella and the latter’s infant child, Miguel. Her other daughters were sent abroad to marry foreign princes, with Catherine of Aragon travelling to England to wed Henry VII’s eldest son, Prince Arthur, and then – after his death – Henry VIII. As the son of the iron-willed Margaret Beaufort, Henry VII was fascinated by the strong Spanish royal women, telling one visitor that he would give up “half of his kingdom” if Catherine was like her mother. Isabella found a natural ally in Henry’s queen, Elizabeth of York, when they exchanged letters about their betrothed children (with the latter advising Isabella to teach her daughter to drink wine, since English water was unpalatable). When Isabella wrote begging the Tudors to be loving towards her daughter, it was Elizabeth – rather than Henry – who took that to heart. “We do not wish our daughter to be the cause of any loss to England. On the contrary, we desire that she should be the source of all kinds of happiness,” Isabella wrote. “We, therefore, beg the King, our brother, to moderate the expenses. Rejoicings may be held, but we ardently implore him that the substantial part of the festival should be his love; that the Princess should be treated by him and by the Queen as their true daughter.”

But it was also Isabella who insisted that, after Arthur’s death, Catherine remain in England to wait for Henry to reach marriageable age. This was an unhappy period for her daughter, made worse by Henry’s penny-pinching, though Catherine eventually began to show some of her mother’s mettle – especially when she was made ambassador for Castile and Aragon. Catherine’s wait was long and she had not yet married Henry VIII when she sat down at Durham House, her palatial house overlooking the Thames at Westminster, to write a letter to her mother on November 26, 1504. She knew that the previous years had been hard on Isabella, who was ill and had not written for a year. Catherine told Isabella that she could not ‘be satisfied or cheerful’ until she saw a letter from her mother telling her that she had recovered. It was too late. That same day, Ferdinand wrote his own letter announcing Isabella’s death. ‘Her passing is for us the deepest grief that could ever happen to us in this life, for we have lost the best and most excellent wife that a king ever had,’ he said. Europe’s first great queen had died. England would provide the next one.