With William dead, the succession to the English throne was thrown into doubt. Rules of succession were uncertain in western Europe at the time; in some parts of France, male primogeniture was becoming more popular, in which the eldest son would inherit a title. It was also traditional for the king of France to crown his successor while he was still alive, making the intended line of succession relatively clear. This was not the case in England, where the best a noble could do was to identify what Professor Eleanor Searle has termed a pool of legitimate heirs, leaving them to challenge and dispute the inheritance after his death. The problem was further complicated by the sequence of unstable Anglo-Norman successions over the previous sixty years. William the Conqueror had invaded England, his sons William Rufus and Robert Curthose had fought a war between them to establish their inheritance, and Henry had only acquired control of Normandy by force. There had been no peaceful, uncontested successions.