The claim of Duke William II of Normandy to the English throne derives from his familial relationship with the childless Anglo-Saxon King Edward the Confessor, who may have encouraged William's royal hopes.

Edward dies in January 1066 and is succeeded by his brother-in-law Harold II of England.

The Norwegian king Harald Hardrada invades northern England in September and is victorious at the Battle of Fulford, but Harold defeats and kills him at the Battle of Stamford Bridge on September 25, 1066.

William lands in southern England within days leading an army of Continental soldiers—Norman, Breton, and French—to invade and occupy England.

Harold, leaving a significant portion of his army in the North, confronts William's invaders on October 14 at the Battle of Hastings; William's force defeat Harold, who is killed in the engagement.

The leader of the Norman Conquest of England becomes known as William the Conqueror.

England’s childless Anglo-Saxon king, Edward, called the Confessor, on his deathbed in the first days of January 1066, allegedly names Harold Godwinson, Earl of Wessex, heir to the English throne; he dies on January 4 or 5.

In any case, the Witenagemot, gathering at Thorney Island where Edward's Westminster Abbey had been dedicated days before he died, proclaims Harold, the richest and most powerful of the English aristocrats, King of England.

He is crowned as Harold II on January 6, by Ealdred, Archbishop of York, probably in the new Westminster Abbey, although Norman propaganda will claim the ceremony had been performed by Stigand, the uncanonically elected Archbishop of Canterbury.

Later in the month, Harold marries Ealdgyth, daughter of the late Earl Ælfgar, and widow of Welsh ruler Gruffydd ap Llywelyn.

Harold is at once challenged by two powerful neighboring rulers.

Duke William of Normandy claims that he had been promised the throne by King Edward and that Harold had sworn agreement to this.

Harald III of Norway, commonly known as Harald Hardrada, also contests the succession.

His claim to the throne is based on an agreement between his predecessor Magnus I of Norway, and the earlier King of England Harthacanute, whereby if either died without heir the other would inherit both England and Norway.

William and Harald immediately set about assembling troops and ships for separate invasions.

Halley's Comet appears in the sky in April 1066, and is widely reported throughout Europe.

Contemporary accounts connect the comet's appearance with the succession crisis in England.

The first sign of real trouble for Harold comes from his exiled brother, Tostig.

According to the Anglo Saxon Chronicle, Tostig lands on the Isle of Wight in May 1066 with a fleet he had recruited in Flanders, later joined by other ships from Orkney, before ravaging the south coast of England, and ending up at Sandwich, Kent.

At Sandwich, Tostig is said to have enlisted and impressed sailors.

Threatened by Harold's fleet, …

…Tostig moves north and raids in East Anglia and Lincolnshire.

He is driven back to his ships by the brothers Edwin, Earl of Mercia, and Morcar, Earl of Northumbria.

Deserted by most of his followers, he withdraws to Scotland, where he will spend the middle of the year recruiting fresh forces.

Meeting and making a pact with Harald Hardrada, King of Norway, he agrees to support Hardrada in his invasion of England.

The medieval historian Orderic Vitalis has a different version of this story; he says that Tostig traveled to Normandy to enlist the help of William, Duke of Normandy.

Then, as William was not ready to get involved at that stage, Tostig had sailed from the Cotentin Peninsula, but because of storms ended up in Norway, and made his pact with Harald Hardrada there.

Edwin had succeeded as Earl of Mercia on the death of his father, Aelfgar, in 1062.

His younger brother, Morcar, had been elected Earl of Northumbria on October 3, 1065, when Tostig Godwinson was ejected by the Northumbrians.

Tostig returns in early September with further forces recruited in Flanders and Scotland, accompanied by King Harald Hardrada, who had set sail from Norway with three hundred ships.

As he approaches the English coast, his fleet is joined by Tostig's ships and they sail together along the River Ouse towards the city of York.

The Orderic Vitalis version says that in the month of August Hardrada and Tostig set sail across the wide sea with a favorable wind and landed in Yorkshire.

Edwin had brought some soldiers to the east to prepare for an invasion by the Norwegians.

The battle starts with the English spreading out their forces to secure their flanks.

On the right flank is the River Ouse, and on the left flank is the Fordland, a swampy area.

The disadvantage to the position is that it gives Harald higher ground, which is perfect for seeing the battle from a distance.

Another disadvantage is that if one flank were to give way, the other one would be in trouble.

If the Anglo-Saxon army is forced to retreat, it will not be able to because of the marshlands.

They will have to hold off the Norwegians as long as possible.

Harald's army approaches from three routes to the south.

Harald lines his army up to oppose the Anglo-Saxons, but he knows it will take hours for all of his troops to arrive.

His least experienced troops are sent to the right and his best troops on the riverbank.

The English strike first, advancing on the Norwegian army before it can fully deploy.

Morcar's troops push Harald's back into the marshlands, making progress against the weaker section of the Norwegian line.

However, this initial success proves insufficient for victory to the English army, as the Norwegians bring their better troops to bear upon them, still fresh against the weakened Anglo-Saxons.

Harald brings more of his troops from the right flank to attack the center, and sends more men to the river.

The invaders are outnumbered, but they keep pushing and shoving the defenders back.

The Anglo-Saxons are forced to give ground.

Edwin's soldiers who are defending the bank are now cut off from the rest of the army by the marsh, so they head back to the city to make a final stand.

Other invading Norwegians, who are still arriving, find a way to get around the thick fighting and open a third front against the Anglo-Saxons.

The defenders, outnumbered and outmaneuvered, are defeated.

Edwin and Morcar however, manage to survive the fight.

It has been estimated that at Fulford the Norwegians had about ten thousand troops, of which six thousand were deployed in the battle, and the defenders five thousand.

Casualties during the battle are heavy on both sides.

Some estimates claim fifteen percent dead giving a total of sixteen hundred and fifty (based on eleven thousand troops being deployed in the battle).

It is clear from all accounts that the mobilized power of Mercia and Northumbria was cut to pieces at Fulford.