| Treaty of peace, friendship and confederation between John I of Portugal and Richard II, King of England | |
|---|---|
 Treaty of Windsor | |
| Drafted | 9 May 1386 |
| Signed | 24 February 1387 |
| Location | Westminster |
| Negotiators |
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| Signatories | |
| Ratifiers | |
| Depositary | Portuguese National Archives |
| Language | Latin |
The Treaty of Windsor is a diplomatic alliance signed between Portugal and England on 9 May 1386 in Windsor and sealed by the marriage of King John I of Portugal (House of Aviz) to Philippa of Lancaster, daughter of John of Gaunt, 1st Duke of Lancaster.[1] With the victory at the Battle of Aljubarrota, assisted by English archers, John I was recognised as the undisputed King of Portugal, putting an end to the interregnum of the 1383–1385 Crisis.[1] The Treaty of Windsor established a pact of mutual support between the countries.[1]
This document is preserved at the Portuguese National Archives.[2]
Historian Matthew Winslett says, "This treaty has been the cornerstone of both nations' relations with each other ever since."[3]
Enduring to this day, the Treaty of Windsor holds the record as the longest-lasting diplomatic treaty in recorded history.[4]
