William de Longespée, 3rd Earl of Salisbury also known as William Longsword, was an illegitimate son of King Henry II. He received his title by marrying the daughter of William of Salisbury, the 2nd Earl of Salisbury.
William was an army commander from 1210-1212. He led a naval fleet in the Battle of Damme. He commanded an army unit in the Battle of Bouvines. He fought in support of King John after the sigining of the Magma Carta, but then switched sides. He also fought in support of John's successor, Henry III.