On June 16, 1373, King Edward III of England and King Ferdinand I of Portugal signed a formal treaty of alliance, the precursor to the enduring Anglo-Portuguese relationship. The alliance, formalized at the Church of São Salvador in Tagilde, was rooted in mutual strategic interests against Castile. This treaty laid the foundation for what would become the longest-standing diplomatic alliance in world history, with profound implications for the subsequent political and military alignment of Europe.

The Anglo-Portuguese Alliance was celebrated in the chronicles and poetry of both nations, 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.