The 1299Treaty of Montreuil (Latin:Tractatus apud Musterolium) orMontreuil-sur-Mer[a] (French:Traité de Montreuil-sur-Mer) was part of the negotiations betweenKing Edward I ofEngland andPhilip IV ofFrance during the 1294–1303Gascon War. It provided for the betrothal of Edward's sonPrince Edward of Caernarfon and Philip's daughterIsabella ("the She-Wolf of France"). It was drafted atMontreuil on 19 June 1299[1] and ratified by Edward I on 4 July[2] and Philip IV on theFeast of the Invention ofSt Stephen (3 August).[3]
The treaty was negotiated on Edward I's behalf byHenry de Lacy, theearl ofLincoln;Guy de Beauchamp, theearl ofWarwick; andAmadeus V,count ofSavoy. Under its terms, should Edward I default on the treaties,[clarification needed] he would forfeitGascony; if Philip defaulted, he would pay a fine of£100,000.[4] It was said by contemporaries that the alliance brought "great unhappiness to both parties".[5] Edward I also privately instructed the count to enquire about Philip's half-sisterMargaret of France, whom Edward married atCanterbury soon afterwards on 10 September.
Upon France's ratification of the original treaty, it was supplemented by theTreaty of Chartres on the same day.[6] The final betrothal of Prince Edward, by then Prince of Wales, and Isabella formed part of the1303 Treaty of Paris that finally concluded the war. Following the prince's accession asEdward II in 1307, the two married atBoulogne-sur-Mer on 25 January 1308, when Isabella was 12.