The Treaty of Windsor established a mutual support alliance between England and Portugal that remains the longest-lasting diplomatic treaty in recorded history.
Key Facts
- Date signed
- 24 February 1387
- Signing location
- Windsor, Berkshire
- Portuguese signatory
- King John I of Portugal (House of Aviz)
- Sealed by marriage of
- John I of Portugal and Philippa of Lancaster
- Archive copies held
- Torre do Tombo, Lisbon; National Archives, London
- Status
- Still in effect; oldest diplomatic treaty in recorded history
Location
Cause → Event → Consequence
Following the 1383–1385 Crisis in Portugal, John I secured his throne with English military assistance, notably English archers at the Battle of Aljubarrota. This victory and the resulting alliance with England required formal diplomatic codification to bind the two kingdoms in mutual support.
The Treaty of Windsor was signed on 24 February 1387 between the Kingdom of Portugal and the Kingdom of England at Windsor, Berkshire. It established a pact of mutual support in international affairs and was reinforced by the marriage of King John I of Portugal to Philippa of Lancaster, daughter of John of Gaunt, 1st Duke of Lancaster.
The alliance endured as a cornerstone of Anglo-Portuguese relations for centuries. Described by Winston Churchill in 1943 as 'an alliance without parallel in world history,' the treaty remains in force today and is recognised as the longest-lasting diplomatic treaty in recorded history.
Political Outcome
Mutual support alliance established between Portugal and England; treaty remains in force and is the longest-lasting diplomatic treaty in recorded history.
John I's legitimacy as King of Portugal was contested during the 1383–1385 interregnum crisis.
John I recognised as undisputed King of Portugal; Anglo-Portuguese alliance formally established.