On May 9, 1386, the Treaty of Windsor was signed between Portugal and England, formally renewing and strengthening the alliance that had been established in 1373. The treaty was sealed by the marriage of King John I of Portugal to Philippa of Lancaster, daughter of John of Gaunt, which cemented the alliance through dynastic ties. The treaty remains the oldest diplomatic alliance in the world that is still in force and was a decisive factor in Portugal's successful resistance to Castilian domination.

The Treaty of Windsor and the marriage of King John I to Philippa of Lancaster were celebrated in Portuguese and English poetry, with poets composing verses that praised the friendship between the two kingdoms. The alliance became a recurring theme in Portuguese literature, particularly in the epic poetry of the Age of Discovery, where the English alliance was seen as a pillar of Portuguese independence. The treaty's literary legacy is also evident in the works of English poets who wrote about Portugal, including the 16th-century poet Edmund Spenser, who celebrated the alliance in his allegorical epic The Faerie Queene.