The 1386 Treaty of Windsor sealed the Anglo-Portuguese ruling alliance

Evidence

Historically, the relationship between the maturing ruling classes of Portugal and England had been sealed at the end of the fourteenth century with the Treaty of Windsor (1386), which secured the Portuguese throne from the ambitions of Castile’s monarchy, and was itself closed by the marriage between Joao of Avis and Philippa of Lancaster, the daughter of John of Gaunt. — Cedric J. Robinson, Black Marxism_ The Making of the Black…, 525