John I of Castile in the context of "John I of Portugal"

The Catholic Monarchs were Queen Isabella I of Castile (r. 1474–1504) and King Ferdinand II of Aragon (r. 1479–1516), whose marriage and joint rule marked the de facto unification of Spain. They were both from the House of Trastámara and were second cousins, as they were both descended from John I of Castile. To remove the obstacle that this consanguinity would otherwise have posed to their marriage under canon law, they were given a papal dispensation by Sixtus IV. They married on October 19, 1469, in the city of Valladolid; Isabella was 18 years old and Ferdinand a year younger. Most scholars generally accept that the unification of Spain can essentially be traced back to the marriage of Ferdinand and Isabella. Their reign was called by W.H. Prescott "the most glorious epoch in the annals of Spain."

Spain was formed as a dynastic union of two crowns rather than a unitary state, as Castile and Aragon remained separate kingdoms until the Nueva Planta decrees of 1707–1716. The court of Ferdinand and Isabella was constantly on the move in order to bolster local support for the crown from local feudal lords. The title of "Catholic King and Queen" was officially bestowed on Ferdinand and Isabella by Pope Alexander VI in 1494, in recognition of their defence of the Catholic faith within their realms.

The Battle of Aljubarrota was fought between the Kingdom of Portugal and the Crown of Castile on 14 August 1385. Forces commanded by King John I of Portugal and his Constable Nuno Álvares Pereira, with the support of English allies, opposed the army of King John I of Castile with its Aragonese and French allies, as well as Genoese mercenaries at São Jorge, between the towns of Leiria and Alcobaça, in central Portugal. The result was a decisive victory for the Portuguese, ruling out Castilian ambitions to the Portuguese throne, ending the 1383–85 Crisis and assuring John's position as King of Portugal.

Portuguese independence was safeguarded and a new dynasty, the House of Aviz, was established. Scattered border confrontations with Castilian troops would persist until the death of John I of Castile in 1390, but these posed no real threat to the new dynasty.

Beatrice (Portuguese: Beatriz, pronounced [bi.ɐˈtɾiʃ]; 7–13 February 1373 – c. 1420) was the only surviving legitimate child of King Ferdinand I of Portugal and his wife, Leonor Teles. She became Queen consort of Castile by marriage to King John I of Castile. Following her father's death without a legitimate male heir, she claimed the Portuguese throne but lost her claim to her uncle, who became King John I of Portugal, founder of the House of Aviz.

During her early years, Beatrice was a pawn in the changing politics of foreign alliances of her father, who negotiated successive marriages for her. She would eventually marry King John I of Castile, by whom Beatrice became Queen Consort of Castile. At the death of her father, Beatrice was proclaimed Queen regnant of Portugal and her mother assumed the regency in her name. Opposition to the regency, fear of the Castilian domination and loss of Portuguese independence led to a popular rebellion and civil war between the late King Ferdinand I's illegitimate brother, John of Aviz, who wrested control of the regency from the dowager queen, and the supporters of Beatrice and her husband, John I of Castile, who claimed the throne of Portugal by right of his wife. In 1385, John of Aviz was proclaimed King of Portugal, and the King of Castile was definitively defeated in the Battle of Aljubarrota, effectively ending any prospects for Beatrice and her husband to assert their rights to the Portuguese crown.

Henry III of Castile (4 October 1379 – 25 December 1406), called the Suffering due to his ill health (Spanish: Enrique el Doliente, Galician: Henrique o Doente), was the son of John I and Eleanor of Aragon. He succeeded his father as King of Castile in 1390.