William's claim to the English throne derived from his familial relationship with the childless Anglo-Saxon King Edward the Confessor, who may have encouraged William's hopes for the throne. Edward died in January 1066 and was succeeded by his brother-in-law Harold Godwinson. The Norwegian king Harald Hardrada invaded northern England in September 1066 and was victorious at the Battle of Fulford, but Harold defeated and killed him at the Battle of Stamford Bridge on 25 September. Within days, William landed in southern England. Harold marched south to confront him, leaving a significant portion of his army in the north. Harold's army confronted William's invaders on 14 October at the Battle of Hastings; William's force defeated Harold, who was killed in the engagement.
Although William's main rivals were gone, he still faced rebellions over the following years and was not secure on his throne until after 1072. The lands of the resisting English elite were confiscated; some of the elite fled into exile. To control his new kingdom, William granted lands to his followers and built castles commanding military strongpoints throughout the land. Other effects of the conquest included the court and government, the introduction of the Norman language as the language of the elites, and changes in the composition of the upper classes, as William enfeoffed lands to be held directly from the king. More gradual changes affected the agricultural classes and village life: the main change appears to have been the formal elimination of slavery, which may or may not have been linked to the invasion. There was little alteration in the structure of government, as the new Norman administrators took over many of the forms of Anglo-Saxon government.
In 911 the Carolingian French ruler Charles the Simple allowed a group of Vikings under their leader Rollo to settle in Normandy as part of the Treaty of Saint-Clair-sur-Epte. In exchange for the land, the Norsemen under Rollo were expected to provide protection along the coast against further Viking invaders. Their settlement proved successful, and the Vikings in the region became known as the "Northmen" from which "Normandy" and "Normans" are derived. The Normans quickly adopted the indigenous culture as they became assimilated by the French, renouncing paganism and converting to Christianity. They adopted the langue d'oïl of their new home and added features from their own Norse language, transforming it into the Norman language. They intermarried with the local population and used the territory granted to them as a base to extend the frontiers of the duchy westward, annexing territory including the Bessin, the Cotentin Peninsula and Avranches.
In 1002 English king Æthelred the Unready married Emma of Normandy, the sister of Richard II, Duke of Normandy. Their son Edward the Confessor, who spent many years in exile in Normandy, succeeded to the English throne in 1042. This led to the establishment of a powerful Norman interest in English politics, as Edward drew heavily on his former hosts for support, bringing in Norman courtiers, soldiers, and clerics and appointing them to positions of power, particularly in the Church. Childless and embroiled in conflict with the formidable Godwin, Earl of Wessex and his sons, Edward may also have encouraged Duke William of Normandy's ambitions for the English throne.
When King Edward died at the beginning of 1066, the lack of a clear heir led to a disputed succession in which several contenders laid claim to the throne of England. Edward's immediate successor was the Earl of Wessex, Harold Godwinson, the richest and most powerful of the English aristocrats. Harold was elected king by the Witenagemot of England and crowned by the Archbishop of York, Ealdred, although Norman propaganda claimed the ceremony was performed by Stigand, the uncanonically elected Archbishop of Canterbury. Harold was immediately challenged by two powerful neighbouring rulers. Duke William claimed that he had been promised the throne by King Edward and that Harold had sworn agreement to this; King Harald III of Norway, commonly known as Harald Hardrada, also contested the succession. His claim to the throne was based on an agreement between his predecessor, Magnus the Good, and the earlier English king, Harthacnut, whereby if either died without heir, the other would inherit both England and Norway. William and Harald at once set about assembling troops and ships to invade England.