The oldest alliance still in force between two nations, binding England and Portugal since 1373 through mutual military and political commitments.
Key Facts
- Date signed
- 16 June 1373
- Signatories
- Edward III of England; Ferdinand I and Leonor of Portugal
- Duration
- Over 650 years, still in effect
- Key reinforcement
- Treaty of Windsor, 1386
- WWII reactivation
- 1943, granting UK Azores air and naval facilities
- 650th anniversary
- Officially commemorated 16 June 2023
By the Numbers
Location
Cause → Event → Consequence
England and Portugal shared strategic maritime interests and a mutual need for alliance against common rivals, particularly Castile and France, motivating both crowns to seek a formal, enduring diplomatic and military partnership in the fourteenth century.
On 16 June 1373, King Edward III of England and King Ferdinand I with Queen Leonor of Portugal signed a treaty in London establishing perpetual friendship, union, and alliance between the two kingdoms, committing each to aid the other against enemies.
The alliance shaped centuries of Anglo-Portuguese cooperation, surviving the Iberian Union's interruption, supporting British operations in the Peninsular War, facilitating neutral coordination during World War II, and being cited as recently as the 1982 Falklands War. It remains the world's longest-standing diplomatic alliance.
Political Outcome
Perpetual alliance established between England and Portugal, still in force as of 2023 and recognised as the world's longest-standing treaty.
England and Portugal lacked a formal mutual-defence framework
Formal perpetual alliance committed both kingdoms to military and political mutual support