William de Longespée, jure uxoris 3rd Earl of Salisbury (c. 1176 – March 7, 1226) was an English noble, primarily remembered for his command of the English forces at the Battle of Damme and for remaining loyal to King John.
He was an illegitimate son of Henry II of England. His mother was unknown for many years, until the discovery of a charter of William mentioning "Comitissa Ida, mater mea" (engl. "Countess Ida, my mother") 
This Ida was further identified as the wife of Roger Bigod, 2nd Earl of Norfolk .
King Henry acknowledged William as his son and gave him the Honour of Appleby, Lincolnshire in 1188. Ten years later, his half-brother, King Richard I, married him to a great heiress, Ela, countess of Salisbury in her own right, and daughter of William of Salisbury, 2nd Earl of Salisbury.
He was with King Richard I in Normandy, 1196-98, and was present at John's coronation 27 May 1199. He served as Sheriff of Wiltshire from 1199-1202, 1203-1207, and from 1215 until his death.
In 1202 he was on a diplomatic mission to France, one of an escort of Llewelyn to a meeting with King John at Worcester in 1204, escorted William the Lion, King of Scots, to his meeting with King John at York in Nov 1206, and headed an embassy in Mar 1209 to the prelates and princes of Germany. He was keeper of the castle of Avranches in 1204, Keeper of the March of Wales in 1209, keeper of Dover Castle, and accompanied the King in his expedition to Ireland in 1210.
He was a commander in the king's Welsh and Irish expeditions of 1210-1212. The king also granted him the honour of Eye.
In May 1213, Salisbury led a large fleet to Flanders, where he seized or destroyed a good part of a French invasion fleet anchored at or near Damme. This ended the invasion threat but not the conflicts between England and France. In 1214, Salisbury was sent to help Otto IV of Germany, an English ally, who was invading France. Salisbury commanded the right wing of the army at their disastrous defeat at the Battle of Bouvines, where he was captured, his release being negotiated in Feb 1214/15.
By the time he returned to England, revolt was brewing amongst the barons. Salisbury was one of the few who remained loyal to John. In the civil war that took place the year after the signing of the Magna Carta, Salisbury was one of the leaders of the king's army in the south. However, after the French prince Louis (later Louis VIII) landed as an ally of the rebels, Salisbury went over to his side. Presumably, he thought John's cause was lost.
He served as sheriff of Somerset and Devon 1216-17, and received a grant of Sherborne Castle and the co. of Somerset in that same year. He was with the Earl Marshal at the relief of Lincoln, and with Hubert de Burgh in the victory over the French fleet off Thanet, and was one of the guarantors of the truce with Louis at Lambeth. In Oct 1223, he was with the King in the successful expedition against Llewelyn, and in 1224 was keeper of the castles of Bridgnorth and Shrewsbury, as well as Sheriff of Salop and Staffordshire 1223-24.
After John's death and the departure of Louis, Salisbury, along with many other barons, joined the cause of John's young son, now Henry III of England. He held an influential place in the government during the king's minority and fought in Gascony to help secure the remaining part of the English continental possessions. Salisbury's ship was nearly lost in a storm while returning to England in 1225, and he spent some months in refuge at a monastery on the French island of Ré. He died not long after his return to England at Salisbury Castle. Roger of Wendover alleged that he was poisoned by Hubert de Burgh. He was buried at Salisbury Cathedral in Salisbury, Wiltshire, England.
William de Longespee's tomb was opened in 1791. Bizarrely, the well-preserved corpse of a rat which carried traces of arsenic , was found inside his skull. The rat is now on display in a case at the Salisbury and South Wiltshire Museum.
2. ed. London, Vera C. M. (1979). Cartulary of Bradenstoke Priory. Devizes: Wiltshire Record Society Publications. xxxv.
3. Reed, Paul C. (2002), "Countess Ida, Mother of William Longespée, Illegitimate Son of Henry II", The American Genealogist 77 (2002): 137
4. Phair, Raymond W. (2002), "William Longespée, Ralph Bigod, and Countess Ida", The American Genealogist 77 (2002): 279–281
5. "Salisbury Cathedral".
6. Weis, Frederick Lewis. Ancestral Roots of Certain American Colonists Who Came to America Before 1700. Lines 30–26, 31–26, 33A–27, 108–28, 122–28 & 122A–28.