Spain in the late fifteenth century was politically fragmented, socially divided, and regionally diverse. Isabella’s marriage to Ferdinand of Aragon in 1469 marked a turning point, creating the foundation for a united monarchy. Yet, before their authority could be firmly established, the monarchs faced significant challenges: internal instability, rival claimants, and the need to consolidate power through both force and administrative reform.

The Situation in Spain in 1469

Political Fragmentation

In 1469, Spain was not a unified kingdom but a collection of territories:

The Iberian Peninsula in 1491, with Castile, Aragon, Navarre, Portugal and the Nasrid Emirate of Granada clearly delineated. This visual underlines the political fragmentation that Isabella and Ferdinand inherited. Relief and rivers are included but can be ignored if focusing purely on political boundaries. Source

• Castile: the largest and wealthiest, but unstable due to a disputed succession.

• Aragon: a confederation of territories (Aragon, Catalonia, Valencia) with distinct legal traditions and institutions.

• Navarre and Granada: separate polities, with Granada still under Muslim rule.

The marriage of Isabella of Castile and Ferdinand of Aragon in 1469 was intended to create dynastic stability, but the kingdoms retained separate laws, currencies, and parliaments.

Crisis in Castile

Isabella’s accession was contested by her niece Juana la Beltraneja, supported by powerful nobles and by Portugal. Castile was politically unstable due to:

• Noble dominance: aristocrats exercised independent power, maintaining private armies.

• Royal weakness: Henry IV’s reign left the monarchy discredited, leading to factional struggles.

• Foreign interference: Portugal and France sought to exploit Castilian instability for their own advantage.

The Campaign to Secure the Throne by 1479

Outbreak of the Succession Crisis

Upon Henry IV’s death in 1474, Isabella claimed the throne, while supporters of Juana la Beltraneja challenged her authority. This succession crisis developed into a civil war (1475–1479).

The War of Succession (1475–1479)

The conflict was both a dynastic struggle and a test of Isabella and Ferdinand’s political skill:

• Support for Isabella: urban elites, many churchmen, and reform-minded nobles.

• Support for Juana: backed by Afonso V of Portugal, who invaded Castile in 1475.

• French involvement: France supported Portugal and sought to undermine the Trastámara alliance.

Key developments:

• Battle of Toro (1476): although inconclusive militarily, politically it was a victory for Isabella, as it demonstrated her ability to rally support and weakened Portuguese resolve.

19th-century depiction of the Battle of Toro (1476), fought during the War of the Castilian Succession. While the tactical result was disputed, Isabella’s faction capitalised on it politically to consolidate support. As an historical artwork, precise uniforms and colours may reflect later artistic conventions rather than exact 1476 realities. Source

• Diplomatic settlements: the Treaty of Alcáçovas (1479) ended the war, with Portugal renouncing Juana’s claim. In return, Castile recognised Portuguese control over Atlantic territories, while Castile retained authority in the Canary Islands.

Consolidation of Rule

By 1479, Isabella and Ferdinand had secured their dynastic position:

• Rival claimants were neutralised.

• Their marriage established the basis for joint rule of Castile and Aragon.

• International recognition consolidated their legitimacy.

Administrative Methods of Consolidation

Strengthening Royal Authority

Isabella and Ferdinand employed administrative reforms to reduce noble power and centralise governance:

• Hermandades (brotherhoods): local militias reintroduced in 1476 to maintain order, curbing noble violence.

Facade of the Posada de la Santa Hermandad in Toledo, associated with the Hermandad’s activities. It illustrates the institutional infrastructure underpinning the monarchs’ efforts to police highways and towns. Source

• Royal Council: reorganised to limit noble influence, with increased reliance on professional lawyers and officials loyal to the crown.

• Cortes (parliamentary assemblies): convened more regularly to secure taxation, though carefully managed to avoid excessive political concessions.

Control of Nobility

The monarchs reduced aristocratic autonomy by:

• Revoking unlawfully acquired lands.

• Limiting the right of nobles to maintain private armies.

• Rewarding loyalty selectively to reinforce the crown’s authority.

Use of Symbolism and Legitimacy

Isabella and Ferdinand presented themselves as defenders of order and religion:

• Promoting the “New Monarchy” ideal, emphasising strong, centralised rule.

• Cultivating the image of being chosen by God, reinforcing dynastic legitimacy.

This approach linked their political success to divine providence and positioned them as agents of stability in a previously fractured realm.

Conclusion of the Campaign

By 1479, Spain had not achieved full unification, but Isabella and Ferdinand had secured the Castilian throne, stabilised the monarchy, and introduced reforms that curbed disorder. Their administrative methods—strengthening central institutions, restraining noble power, and promoting law and order—established the framework for the eventual consolidation of Spain under the so-called Catholic Monarchs.