News of the attack on and burning of Scarborough around 15 September 1066 by the Viking invaders under King Harald Hardrada, King of Norway, and Tosti the expelled Earl of Northumberland was speedily brought to King Harold Godwinson who was in London. He had to make the difficult choice of whether to remain in the south to face the threat from William the Bastard, Duke of Normandy or go north to support his brothers-in-law, Earls Edwin and Morcar against the invasion of his brother, Tosti, with his ally and claimant to the English throne, Harald Hardrada.

King Harold decided to go north to settle the matter of the invasion at hand. Once that threat had been dealt with, he could make a quick return to the south to guard against Duke William's threatened invasion. It is apparent from this decision that Harold was not aware that William was poised to sail, and the only thing that prevented him from doing so was the unfavourable wind. Probably Harold had heard about William's fleet being mauled during the Channel storm of 12 September, which had destroyed part of his own fleet. There is also the hinted possibility that the fleets had clashed prior to the storm. This and the fact that the autumn storm season was about to start may have been influential in him deciding to go to Yorkshire and deal with the Viking threat.

On 20 September, the day of the Battle of Fulford, Harold and his brother Gryth set out with their Huscarls and royal Thegns for Yorkshire. The mounted force met up with elements from the West Mercian and East Anglian Fyrd on the way north. The road distance from London to York is approximately 320 km. On Sunday, 24 September, only four days after leaving London, the army rode into Tadcaster, which is about 16 km south-west of York.  They had averaged an incredible 80 km per day!

Tadcaster was situated very strategically for Harold. At this point, the Roman road that Harold had taken north was very close to the south bank of the Wharfe, only 3.5 km from where Hardrada had left his fleet. Harold quickly learned of the defeat suffered by the Earls Edwin and Morcar at Fulford. Harold then sealed off Tadcaster, and placed guards at other strategic points on the way to York. Secure in the knowledge that his presence in the area was unknown and that he had taken all necessary precautions to keep it that way, he allowed his army to rest and recuperate in the meadows around Tadcaster and continued to gather intelligence.

Morcar's sailors, whose small fleet was moored at Ulleskill, informed him that Hardrada and Tosti, and their entire army, had left York and returned to their fleet at Riccall. People who had been in York when it surrendered to Hardrada informed him that the city of York had been compelled to give supplies and hostages to the Vikings. He also heard that the Northumbrians had been commanded to bring additional supplies and 100 hostages to the Vikings at Stamford Bridge.

Based upon the information he received, Harold decided that his best course of action was to go to York early the next morning, re-establish his authority there, and then take his army to Stamford Bridge, where his sudden and unexpected appearance would catch Harald Hardrada and Tosti off guard.

King Harold and the army departed Tadcaster on Monday morning, 25 September and rode to York. From there, with the few armed men the city could provide, he proceeded to Stamford Bridge, which is about 9 km or so to the north-east. Harold's army kept to the Roman road then halted at Helmsley Gate, just out of sight of Stamford Bridge. There they waited as the Vikings leisurely arrived for the meeting. So relaxed were the Norse that many sunbathed whilst others went swimming in the river Derwent. When he determined the time was right, Harold ordered the Huscarls and Fyrdmen forward.

As Harold's army came over the rise on the hill and the sun reflected off their helmets, spears and long axes, the Vikings were stunned. The complete surprise that Harold wanted had been achieved. As they were only expecting a hand over of supplies and hostages, Harald Hardrada, Tosti, and the Viking army had divested themselves of their armour, which they had sent back to the ships. Adding to their woes was the fact that they had left about a third of the 5,000 men remaining to them after the fight at Fulford to guard the ships.

The Vikings were caught with their force divided with a smaller group on the northern end of the bridge with the English bearing down on them, whilst the larger part was on the southern end.

Harald Hardrada did not know the lay of the land and had no plan in mind because he had not expected a battle. King Harold's rebel brother, Tosti, had been Earl of Northumberland for 10 years and was well acquainted with the area and knew his brother's fighting capabilities first-hand. He urged Hardrada not to accept battle but to immediately retreat as quickly as possible back to Riccall. Hardrada considered this, but decided to give battle because he knew Harold's Huscarls would surely close in upon his flank and rear before long. He may also have reasoned that Harold would have sent a small force ahead to Kexby, to secure the essential crossing of the Derwent at that place, thereby blocking any further retreat to Riccall.

Deciding to accept battle, Hardrada quickly dispatched a messenger back to Eystein Orre, the commander of the contingent he had left behind at Riccall, ordering him to bring up the other third of the army to Stamford Bridge as quickly as possible. Then, he quickly evacuated most of his men who were on the north bank, to the south bank, leaving a small rear-guard to hold the bridge.  Hardrada and Tosti had just enough time to form their best armed men into a line of defence along a ridge about 300 yards south-east of the Derwent.

The rear-guard at the bridge was not able to hold out long against the English and was quickly overwhelmed, though one saga tells that after his comrades had been killed, a lone berserk held the English at bay. An English warrior got into a boat and had himself rowed under the bridge. Through a gap in the planks he stuck the berserk with his spear.

Once over the bridge, the English formed themselves into a long line several ranks deep, facing the enemy deployed along the ridge above them.

King Harold rode up and offered his brother, Tosti, his earldom back if he would lay down his arms and join him, possibly knowing that the offer was unlikely to be accepted, but knowing that, if it were that it were, it would damage the Viking army's morale. Tosti asked what English lands Harald of Norway could expect if he dismissed the army. King Harold's reply was that as Harald was taller than most, he would grant him seven foot of good English soil!

After the entire English army had crossed the bridge and deployed into the line, King Harold ordered them forward. The Vikings lack of armour would have counted against them. The English were also at a disadvantage having had a19 km march that morning from Tadcaster to Stamford Bridge. The slaughter on both sides was high and the battle was balanced, when first Harald, and then Tosti fell. Even though both of their leaders were killed, the Vikings fought on in small groups. They were still valiantly defending themselves when Eystein Orre arrived with the reinforcements. However, these troops proved to be of no assistance. The forced march they had been compelled to make on this unusually hot day had spent their energy, and they too, were quickly overwhelmed and Eystein Orre was killed.

The Battle of Stamford Bridge was a resounding victory for King Harold and an overwhelming defeat for the Norse. The Vikings who survived the battle straggled back to Riccall under hot pursuit of the English. There was a last-gasp defence at the fleet but this was quickly ended and the surviving Vikings capitulated to King Harold. Desiring total victory but not maddened with blood-lust to kill all the enemy, Harold rounded up all the surviving Vikings and let them go. Amongst the captives was Olaf, Harald Hardrada's son. Of the 360 or so ships that brought the invaders, only 36 were needed to take the survivors home!