On September 21, 1371, John of Gaunt, Duke of Lancaster and son of King Edward III of England, married Constance of Castile, the daughter of the deposed King Pedro I of Castile. Through this marriage, John of Gaunt laid claim to the Castilian throne, a move that would complicate English foreign policy for years to come. The marriage was part of a broader strategy to expand English influence in Iberia and challenged the legitimacy of the newly established Trastámara dynasty.

John of Gaunt's marriage and his claim to Castile became a subject of political poetry and propaganda in both England and Castile, with poets on both sides of the conflict using verse to assert their patrons' legitimacy. The marriage was celebrated in English court poetry, which portrayed John of Gaunt as a chivalric hero and rightful king of Castile, while Castilian poets composed verses that ridiculed his pretensions. This poetic war of words reflected the broader cultural and political tensions of the Hundred Years' War and prefigured the themes of legitimacy and dynastic ambition that would dominate English and Spanish poetry for generations.