During the 9th and 10th centuries, continually threatened by Viking invasions, France became a very decentralized state: the nobility's titles and lands became hereditary, and the authority of the king became more religious than secular and thus was less effective and constantly challenged by powerful noblemen. Thus was established feudalism in France. Over time, some of the king's vassals would grow so powerful that they often posed a threat to the king. For example, after the Battle of Hastings in 1066, William the Conqueror added "King of England" to his titles, becoming both the vassal to (as Duke of Normandy) and the equal of (as king of England) the king of France, creating recurring tensions.