The succession to Henry I was altered by the death of his son, William Adelin. Left without male heirs, Henry took the unprecedented step of making his barons swear to accept his daughter Empress Matilda, widow of Henry V, Holy Roman Emperor, as his heir. However, her cousin, Stephen of Blois (the third son of Adela of Normandy) gained the support of the barons and usurp the throne, claiming that Henry had changed his mind on his deathbed. This act provoked a seventeen-year long civil war between him and the forces loyal to Matilda.