Aethelred the Unready died on 23 April 1016 after an illness that had dragged on since the previous year. His reign had been disastrous - decades of failed responses to Viking raids, the massacre of Danes in 1002, the great invasions of Sweyn Forkbeard and the long retreat of English authority. Two assemblies rushed to name a successor. The citizens of London proclaimed Aethelred's son Edmund king. The larger Witan, meeting at Southampton, proclaimed Cnut. From spring to autumn the two kings fought five separate battles across southern England - at Penselwood, Sherston, the Thames at Brentford, Otford, and finally Assandun - in a campaign that historians have called one of the most remarkable military efforts of any English king. Edmund earned his nickname Ironside in those months. But by October the war had to end, one way or another, and Cnut's fleet had returned from a raid into Essex and lay vulnerable at anchor.

Edmund caught the Danes as they were returning to their ships. He drew up his army in three lines and - in a manner that was unusual for kings even then - took his place in the front rank to encourage his men. Cnut, who was more of a strategist than a warrior, did not. The first phases of the battle reportedly went in Edmund's favour. The Anglo-Saxon shield wall, with its huscarls and West Saxon thegns and Edmund himself in the front, pressed the Danish line. And then, midway through the engagement, Eadric Streona broke off and led the Mercian contingent away from the field. Whether he panicked, or had been bribed, or had calculated that Cnut was about to win - the chroniclers blamed treachery, and Eadric's reputation was already dark - the result was the same. The gap he left in the English line was unfillable. The Danes broke through. The English army disintegrated.

Among the English dead were two figures the chroniclers named with particular grief. Ulfcytel was a leading East Anglian thegn who had fought the Danes a dozen times and was so feared they reportedly called East Anglia 'Ulfcytel's land'. He died fighting at Assandun. The second was Eadnoth the Younger, Bishop of Dorchester on Thames, who according to the Liber Eliensis was killed while in the act of saying mass on the field on behalf of Edmund's men. The same source records that his hand was cut off first - to take a ring - and his body afterwards cut to pieces. The detail, whether literal or stylised, was meant to mark the day as one of unholy violation: a bishop killed at the altar, by men who were soon to be sworn to a Christian king. The killing of clergy in battle was deeply taboo in eleventh-century Christendom, and the chroniclers' insistence on the manner of Eadnoth's death tells us as much about what shocked them as about what happened.

After the defeat at Assandun, Edmund Ironside had nothing left to bargain with. He met Cnut at Alney, on an island in the Severn near Deerhurst, and there the two kings divided England between them. Cnut would rule everything north of the Thames - Mercia and Northumbria. Edmund would keep Wessex, the ancestral heart of his dynasty. When one of them died, the other would inherit the whole. It was the kind of treaty that postponed a war rather than ending one. As it happened, postponement was unnecessary. On 30 November 1016, scarcely six weeks after Assandun, Edmund Ironside died at Oxford. He was twenty-six or so. The cause was never recorded - illness, wounds, possibly murder; the chroniclers shrugged. Cnut took the rest of England, exactly as the treaty stipulated, and ruled it for almost twenty years as part of a North Sea empire that also included Denmark and (later) Norway.

On 18 October 1032 - sixteen years to the day after the battle - Cnut consecrated a church at Assandun to commemorate the dead. Which church it was is one of the strands historians use to argue the location of the battlefield. The traditional candidate has long been St Andrew's at Ashingdon, near Rochford in south-east Essex, which sits on a low hill that could plausibly have hosted the battle. But there are also strong arguments for Ashdon in north-west Essex, where finds of Roman and Anglo-Saxon coins are abundant and where, in the 1860s, the construction of the Saffron Walden to Bartlow railway through a field locally known as 'Red Field' uncovered a very large number of human skeletal remains. An attractive candidate for Cnut's commemorative church on that theory is St Botolph's at Hadstock, which is known to date from the early eleventh century and is still largely standing. The dispute will probably not be settled.

The battle has no surviving Anglo-Saxon poem - The Battle of Maldon survived from 991, but no equivalent for Assandun survives. What does survive is a single verse of skaldic poetry quoted in the Knytlinga saga, composed by Ottar the Black, one of Cnut's court poets. Skaldic verse was the formal court praise-poetry of the Viking world; Ottar's lines for Cnut had to please a king who knew he was being praised. The verse is short, technical, and gives the Danish view: Cnut's victory at Assandun is named alongside his other conquests as part of the rolling-up of the North Sea. Read against the chronicles, it is striking how minor the battle seems from the Scandinavian side - one fight among many in a winning campaign - and how absolutely catastrophic it was from the English. The next century of English politics, the Norman Conquest itself in 1066, the whole tangled inheritance question that gave William the Conqueror his pretext - all of it flows downstream from Edmund Ironside losing a battle on an Essex hillside in October 1016.