Who were Ferdinand and Isabella: The husband and wife who transformed medieval Spain?

Ferdinand and Isabella began to rule at a time when rival factions, foreign alliances, and disputed claims to the throne created widespread instability across the Iberian kingdoms.

Following the death of her half-brother, King Henry IV of Castile, in 1474, Isabella had claimed the Castilian crown.

However, her niece Juana, backed by Portugal, also asserted her right to rule. The ensuing conflict, known as the War of the Castilian Succession, lasted from 1475 to 1479 and drew in Portuguese troops and Castilian nobles who feared Isabella’s growing power.

The conflict ended in 1479 with the Treaty of Alcáçovas, which recognised Isabella’s right to rule as Queen of Castile and limited Portuguese influence in Iberian politics.

Under the treaty, Portugal retained control of the Azores, Madeira, and the right to explore the African coast, while Spain held onto the Canary Islands.

That same year, Ferdinand had succeeded his father, John II, as King of Aragon, completing the dynastic union between the two most powerful crowns of Spain.

Although the kingdoms retained their own laws, currencies, and institutions, the monarchs ruled jointly and issued decrees under both their names, allowing them to project an image of unity without immediately dismantling existing systems of government.

Their motto was "Tanto monta, monta tanto, Isabel como Fernando."

In 1494, Pope Alexander VI formally granted them the title “Catholic Monarchs” in recognition of their conquest of Granada two years earlier.

This gave their authority a religious foundation that appealed to their people and to foreign powers.

For this reason, they could often justify forceful domestic policies and international aims as part of a divine mission. 

They had spent the early years of their reign rebuilding royal authority after decades of feudal conflict and civil unrest.

When they removed rebellious nobles from key positions and appointed royal officials to run local administration, they reduced the influence of hereditary aristocracy, and as a result, their government achieved greater stability and consistency across the peninsula. 

One of the most defining military campaigns of their reign unfolded in the mountainous terrain of southern Spain, where the Nasrid Kingdom of Granada had survived for centuries as the last Muslim stronghold in Iberia.

In 1482, the Catholic Monarchs had begun a long and careful war to reclaim this territory, and they directed a well-resourced and coordinated assault that used advances in artillery and siegecraft to serious effect.

For example, their use of gunpowder weapons at sieges such as Loja and Malaga increased their ability to break fortified cities without prolonged attrition.

Papal support helped them call the campaign a crusade, granting indulgences and special taxes to support the effort. 

Isabella had managed supplies, had raised funds, and had rallied support from across Castile, while Ferdinand led troops on the ground and maintained morale through a combination of personal leadership and tactical discipline.

At the same time, they weakened Granada’s political cohesion by offering favourable terms to defecting Muslim leaders, and they helped prevent outside Islamic states from sending military aid.

For a time, Muhammad XII (who was also known as Boabdil), who was the heir to the Nasrid throne, even acted as a vassal of Castile after he was captured in 1483 and released under oath.

Over the course of a decade, many towns and fortresses had fallen one by one, and Christian forces gradually closed in on the city of Granada.

The final surrender took place on 2 January 1492, when Muhammad XII handed over the keys of the Alhambra Palace and accepted exile.

Granada, which was then a city of approximately 50,000 to 80,000 people, lost its independence and its religious protections. 

The fall of Granada completed a major stage of the Reconquista and allowed the monarchs to claim the unification of Christian Spain.

More importantly, it gave them political capital to enforce their vision of a Catholic state.

As a result, Granada’s conquest became the turning point in their religious and imperial aims. 

Consolidating their authority over the nobility and urban elites required persistent effort and administrative innovation, sometimes backed by targeted state violence to secure control.

Ferdinand and Isabella re-established the Santa Hermandad in 1476, which was a royal police force that was funded by towns and that was used to suppress disorder in rural areas, especially where noble private armies had terrorised local populations, and as a result of their activities, levels of violence and robbery dropped sharply in many areas of Castile’s countryside. 

They also turned to educated legal experts known as letrados, who had trained at universities such as Salamanca and Valladolid and who were generally loyal to the Crown.

For this reason, the monarchs could bypass local noble families and instead rely on professional administrators to run the Council of Castile and issue decrees in their name.

The council also acted as a high court, which processed appeals from across the kingdom and helped the monarchs to centralise legal authority.

Some key nobles, such as the Duke of Medina Sidonia, lost practical power, though they retained their titles. 

Ferdinand had to proceed more carefully in Aragon due to stronger local legal traditions.

However, he still gradually increased royal power when he appointed loyal viceroys, negotiated favourable outcomes in the regional courts, and placed trusted men in key positions.

At the same time, he made effective use of patronage to win over local elites. 

Their combined efforts gradually shifted power from the aristocracy to the monarchy by reducing the practical authority of nobles rather than destroying noble titles.

Therefore, by the end of their reign, both monarchs had, in many respects, transformed their courts into effective tools of centralised government. 

The monarchs believed that political unity could not be achieved without religious uniformity.

As early as 1478, they secured a papal bull from Pope Sixtus IV allowing them to establish a Spanish Inquisition under their direct control.

Unlike earlier inquisitions, which operated under the Church, the Spanish version answered to the Crown and became a tool of royal policy. 

It focused especially on conversos, Jews who had converted to Christianity but were suspected of practising Judaism in secret.

The Inquisition used harsh interrogations, secret denunciations, and public punishments to uncover and punish religious deviation.

By 1480 tribunals had begun work in Seville, and within a few years the system had expanded into several parts of both Castile and Aragon. 

Tomas de Torquemada, who was appointed as the first Grand Inquisitor, used his authority to impose strict control over religious life.

For example, he introduced largely standardised procedures, travelled extensively, and oversaw hundreds of investigations.

The Inquisition confiscated property, imprisoned suspects for years, and conducted autos-da-fé, which were public ceremonies in which heretics were sentenced and sometimes executed.

Estimates suggest that roughly 2,000 people were executed between 1480 and 1530 during the early decades of the Inquisition. 

The monarchs defended the institution as a necessary safeguard for the Christian faith.

However, they also used it to remove political opponents, confiscate wealth, and assert control over towns with histories of religious diversity.

Therefore, the Inquisition fulfilled both spiritual and strategic goals. 

After the conquest of Granada, the monarchs issued the Alhambra Decree in March 1492, which gave all unconverted Jews four months to leave Spain.

The decision followed years of pressure from clergy and court factions who believed that Jews were corrupting Christian converts.

By removing them, the Crown hoped to strengthen the purity of the faith. 

Estimates suggest that between roughly 40,000 and 80,000 Jews left Spain as a result of the order.

Some converted to remain, but others fled to North Africa, Italy, or the Ottoman Empire, where Sultan Bayezid II welcomed them.

Their departure left gaps in professions such as medicine, banking, and trade. For example, major urban centres such as Toledo and Seville lost entire communities that had supported local economies for generations.

The royal court ignored appeals from leading Jewish figures such as Isaac Abravanel, who had tried to prevent the expulsion. 

Muslims initially received more lenient treatment under the terms of Granada’s surrender, which officially promised them religious freedom and protection.

However, by 1502, Isabella revoked these rights and ordered all Muslims in Castile to convert or leave.

After her death, Ferdinand imposed similar restrictions in Aragon, which completed the process of forced conversion. 

The monarchs justified these actions as part of their duty to defend Catholicism.

At the same time, they removed potential sources of dissent, seized property, and reinforced the idea that loyalty to the Crown required religious conformity, and as a result, Spain became one of the most religiously homogeneous states in Europe by the early sixteenth century. 

After years of rejection by other courts, Columbus approached Ferdinand and Isabella with his plan to reach Asia by sailing west, and he first presented his proposal to the Spanish court in 1486.

At first, some of their advisors dismissed him, and they argued that his estimates of the Earth’s size were flawed.

However, Isabella became interested once the war with Granada ended and the possibility of new Christian converts abroad appeared attractive. 

They had agreed to fund his expedition in the Capitulations of Santa Fe, signed in April 1492.

He would receive three ships, supplies, and titles in any new lands he discovered.

When he set sail on 3 August 1492 and sighted land on 12 October, he believed he had reached islands off Asia.

However, the monarchs soon realised the implications of his discovery and moved to secure their claims. 

They had requested papal backing and had negotiated with Portugal in the Treaty of Tordesillas (1494), which divided new discoveries along an agreed line.

Although earlier papal bulls issued by Alexander VI influenced the negotiations, the final treaty terms were agreed directly between the two crowns.

The monarchs saw Columbus’s success as a way to extend Catholic influence, access new resources, and compete with Portugal’s maritime empire, and therefore their support largely showed both religious aims and political calculation. 

The monarchs had used marriage to build alliances that fulfilled long-term strategic goals.

Specifically, their children married into the royal families of England, Portugal, and the Habsburg Netherlands.

For example, their daughter Catherine married Prince Arthur of England and, after his death, Henry VIII.

Their daughter Joanna married Philip of Burgundy, which later allowed their grandson Charles to claim vast European territories.

Another daughter, Isabella, married Manuel I of Portugal. 

Also, Ferdinand had pursued military expansion in Italy by asserting his claim to the Kingdom of Naples.

After initially cooperating with France, he broke the alliance and launched his own campaign, and Spanish forces under Gonzalo de Córdoba won a key victory at the Battle of Cerignola in 1503, one of the first battles won by gunpowder infantry, which gave Aragon control of Naples and challenged French desires in the region. 

At the same time, Ferdinand used the language of crusade to justify military actions in North Africa and diplomatic interventions in the Mediterranean.

He promoted himself as a defender of the faith and protector of Europe, and he strengthened Spain’s position within Christendom.

The arrival of precious metals from the Americas had transformed the Spanish economy.

Large shipments of gold and silver from the Caribbean and, later, from Mexico and Peru, significantly increased royal revenue and allowed the Crown to finance wars and government reforms.

For example, the establishment of the Casa de Contratación in 1503 gave the monarchy direct control over trade, taxes, and exploration licences.

New crops such as maize, potatoes, and cacao reached European markets, while goods like sugar and tobacco became important commodities.

However, much of the wealth came from mining operations that relied on forced Indigenous labour, and the encomienda system allowed settlers to extract tribute and work from native populations under the justification of Christian instruction.

Figures such as Bartolomé de las Casas later condemned the system for its brutality. 

As a result, many Indigenous communities suffered catastrophic population loss and widespread cultural destruction that forced mass displacement.

Spanish settlers became wealthy landowners, while the monarchy benefitted from colonial income.

The empire began to shift its focus westward, and Spain’s identity became tied to overseas expansion. Inflation from the influx of precious metals eventually harmed Spain’s economy. 

Ferdinand and Isabella never fully saw the extent of the empire their policies had created; however, they unified the crowns and enforced religious conformity, and they invested in exploration that opened new routes and territories.

These actions helped lay the foundations for a global Spanish empire that would dominate the sixteenth century.