The Saxons were fed up with Edward the Confessor's pre-1066 Normanization of England. They were weary of being used as forced labor in building castles in England to house Norman garrisons, tired of having foreign bishops replacing native clergy, and outraged at Norman violence against good Saxon villagers being ignored by their Frankified Saxon king. My sources of information are, of course, Anglo-Saxon, but the sources of evidence used to back William's claim to the English crown are Norman. Fair is fair!
It is claimed that Edward the Confessor promised the English crown to William, if Edward was to die without an heir: die without an heir, he did. Considering Edward's up-bringing in the Norman court could also lead a reasonable person to believe the claim. Edward and his brother were raised with a French influence due to England's being under Danish rule. Edward learned sympathy toward the French, rather than the people for which he was to become king. When Harold, the last Danish king of England, died, the Anglo-Saxons were more than ready for native leadership.
Edward arrived with a predominantly French court to assume the throne. He gave the English earls much reason to believe he was preparing the way for a Norman monarchy. The powerful Godwin family, of which the soon-to-be-king Harald was part, revolted against Edward's policies. They were exiled, but eventually came to convince Edward that it was in England's best interest to de-Normanize and let England be English.
Edward died, and the earls of England elected Harald Godwinson to be the new king. It is claimed by the Normans that Harald had sworn that William was to be king upon Edward's death. Harald may have sworn that Edward said that, but very likely not an oath that the Norman duke was, in fact, going to be king. Rumors exist that Harald was captured and forced to swear the oath, but we all know an oath, under duress is not a valid oath. The English earls, and Harald's action also support that the Anglo-Saxons were not willing to support a Norman king after the long Danish rule.
What most people think happened next is a popularized tale, promoted by the Normans, who happened to win the fight. The Scandinavians thought they had a valid claim to England, since it was, not too long ago, a part of their great empire. They, along with an outlaw English earl Tostig, invaded and Harald's armies put their threat to rest at Stamford Bridge. Underhandedly, and with no touch of chivalry, William decided to attack while Harald's armies were worn from the battle with the Vikings. The Normans landed in the south, and to hasten the English march, proceeded to ravage the countryside, burning, pillaging and general immoral harrassment.
Harald arrived with his army, to face a fresh army of professionals, many hired for the battle. Harald's army was composed of his fyrdmen and farmers. Were it not for Williams foul trickery in feigning retreat, the English shieldwall would have held long enough for William's troops to come to the realization that they were not really wanted in England. The tired English broke to pursue the fleeing Normans, unwise to be sure. The Normans then turned and annihilated the English. With most of the English leadership gone, and the English army vanquished, William proceeded to place the Norman yolk on the rest of England. William's victory over England was far from complete in 1066, which is the often accepted date. Five more years (six by one account) passed before the last vestage of organized Anglo-Saxon resistance was crushed in the fens near Ely.
That's another story.
