William de Valence, 1st Earl of Pembroke

William de Valence (died 18 May 1296), born Guillaume de Lusignan, was a French nobleman and knight who became important in English politics due to his relationship to Henry III. He was heavily involved in the Second Barons' War, supporting the King and Prince Edward against the rebels led by Simon de Montfort. He took the name de Valence ("of Valence").

He was the fourth son of Isabella of Angoulême, widow of king John of England, and her second husband, Hugh X of Lusignan, Count of La Marche, and was thus a half-brother to Henry III of England, and uncle to Edward I. William was born in the Cistercian abbey in Valence, Couhé-Vérac, Vienne, Poitou, near Lusignan, sometime in the late 1220s (his elder sister Alice was born in 1224).

The French conquest of Poitou in 1246 created great difficulties for William's family, and so he and his brothers, Guy de Lusignan and Aymer, accepted Henry III's invitation to come to England in 1247. The king found important positions for all of them; William was soon married to a great heiress, Joan de Munchensi or Munchensy (c. 1230 – after 20 September 1307), the only surviving child of Warin de Munchensi, lord of Swanscombe, and his first wife Joan Marshal, who was one of the five daughters of William Marshal, 1st Earl of Pembroke and Isabel de Clare, 4th Countess of Pembroke suo jure. As an eventual co-heiress of the Marshal estates, Joan de Munchensi's portion included the castle and lordship of Pembroke and the lordship erected earldom of Wexford in Ireland. The custody of Joan's property was entrusted to her husband, who apparently assumed the lordships of Pembroke and Wexford between 1250 and 1260.

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