Æthelred chose this moment of crisis to fall sick and utterly failed to offer his divided kingdom any inspiring leadership. Edmund raised forces in the north and Eadric raised an army in the south. The two agreed to collaborate, but they did not trust each other and separated without engaging the Danes. Eadric then changed sides, going over to Cnut and taking a unit of Danish mercenaries with him. Edmund’s efforts to organise an effective defence against Cnut came to naught because no one was willing to take the field unless the king himself would lead them. Æthelred, now becoming increasingly frail, refused to leave the safety of London.
Æthelred finally died on 23 April 1016 and was buried in St Paul’s. Such magnates as were present in London, together with the people of the city, chose Edmund as their new king. Edmund had the appetite for war that his father had so conspicuously lacked and he inspired the English to renew their resistance to the Danes. Two weeks after Æthelred died, Cnut’s fleet sailed up the Thames to Greenwich. Edmund hurriedly left to raise forces in Wessex while the Londoners prepared to resist the Danes once again. The widowed queen Emma remained in London with her sons throughout the siege, encouraging the defenders. The Danes could not get past London Bridge so they dug a canal around Southwark and dragged their ships to the west side of the bridge, then surrounded the city with a ditch to prevent anyone getting in and out, but the townsfolk continued to resist fiercely. Edmund’s presence in Wessex forced Cnut to go after him, leaving only a small force to maintain the siege of London. Edmund and Cnut pursued each other across the breadth of southern England, fighting two bloody battles, at Penselwood and Sherston before midsummer. Though inconclusive, they allowed Edmund to relieve the siege of London, but the Danes escaped in their ships. Edmund won a battle at Brentford but not decisively enough to prevent the Danes making another assault on London from both the river and from land. Once again, London put up a stout defence and the Danes withdrew, setting off on a march that took them in a great loop through southern Mercia, south into Wessex and east to Kent. Edmund caught up with them again at Otford, where he inflicted another reverse on the invaders. Edmund was beginning to look as if he might succeed in defeating Cnut. Ealdorman Eadric predictably offered to change sides again and Edmund took him back into favour. ‘No greater folly was ever agreed to than that was,’ said the Anglo-Saxon chronicler, plainly believing that it was safer to have Eadric as an enemy than a friend. Later chroniclers, such as Henry of Huntingdon and William of Malmesbury, claim that Eadric changed sides with Cnut’s agreement so that he would have the opportunity to betray Edmund.
In the autumn, Cnut’s army went into Essex. Edmund gathered his forces and clashed with Cnut at Ashingdon on the morning of 18 October. As soon as the battle started Eadric began to spread alarm and defeatism and took flight with the Mercian levies. With Edmund in command, Eadric’s betrayal was not enough to precipitate a general rout. The rest of the English stood their ground and fought until long after dark, suffering very heavy casualties, including the stalwart ealdorman Ulfcytel of East Anglia. Under cover of darkness, Edmund was able to disengage in reasonably good order and he retreated to Alney in Gloucestershire. Here Eadric presented himself again, urging Edmund to reach a negotiated settlement with Cnut. After such a punishing campaign, Edmund probably had little choice but to agree. By the Treaty of Alney, Cnut and Edmund divided England between them. Edmund kept Wessex, Cnut was granted everything north of the Thames. Only now did the Londoners finally submit to Cnut and paid him unspecified tribute for peace. Cnut moved his fleet to London and took up winter quarters there with his army.
On 30 November 1016, a matter of weeks after agreeing to the division of the kingdom, Edmund Ironside died. No contemporary source says what caused Edmund’s death but later chroniclers, all too plausibly, blamed Eadric. Eadric is alleged to have concealed an assassin – in one version, his own son – in the pit of the king’s privy. When Edmund went to empty his bowels, the assassin struck, stabbing him from below. After Edmund’s death Cnut, by agreement, became king of all of England. There was no opposition; English resistance was broken and leaderless. Significantly, Cnut chose London for his coronation, rather than Winchester or any other place associated with the Wessex dynasty. The surviving English claimants to the throne, King Edmund’s brother Eadwig and Emma’s two young sons by Æthelred, Edward the Confessor and Alfred, went into exile in Normandy. Emma probably went with them. Eadric advised Cnut to kill Edmund’s infant sons, Edward the Exile and Edmund, but he sent them into exile in Sweden and from there they went to Kiev and, eventually to Hungary. Several high ranking English nobles were executed as were some of Cnut’s own commanders. Cnut divided England into four. Keeping Wessex for himself, he left Eadric with Mercia, and rewarded Thorkell the Tall with East Anglia and jarl Erik with Northumbria.
By the end of 1017, Cnut felt secure on his throne and he decided that he no longer needed the dubious benefit of Eadric’s support. At Christmas that year Cnut summoned Eadric to London. Eadric expected to be further rewarded for helping Cnut win the throne but the king upbraided him for his treachery and ordered jarl Erik to cut his head off, ‘so that soldiers may learn from this example to be faithful, not faithless, to their kings’. Cnut could now pay off his army. To do this he levied an exceptionally heavy tribute on the English of 82,500 pounds (37,421 kg), to be paid at Easter 1018. London’s share of the tribute was 10,500 pounds (4,763 kg), a sign of just how important and wealthy the city was becoming. Many of Cnut’s commanders were rewarded with lands and titles, establishing a new Anglo-Danish aristocracy, but there was no large scale Danish settlement as there had been in the ninth century. Cnut’s warriors were mercenaries serving for pay and now that the campaign was over most went home. Cnut retained forty ships’ crews as his housecarls (huskarlar), his household warriors. They were paid by continuing to levy Æthelred’s heregeld. Despite a heavy burden of taxation, Cnut’s rule was not unpopular in England. He made few changes to the traditional institutions of the country and the English welcomed the peace and security he brought after so many years of violence and instability.
After his coronation, Cnut began to negotiate an alliance with Richard of Normandy, which resulted in his marriage to Queen Emma in 1018. Cnut’s main motive for the marriage was to prevent Æthelred’s sons seeking Norman support against him, but he may also have seen her experience as queen of England as an advantage. During his campaigns in England, Cnut had already acquired an English consort in the shape of Ælfgifu of Northampton, with whom he already had two sons, Svein Alfivason and Harold Harefoot. Although he put Ælfgifu aside so that he could marry Emma, Cnut continued to show her favour and she continued to be influential throughout his reign. Later in 1018, Cnut’s brother Harald died and the following year he sailed to Denmark to claim the throne. Cnut always recognised that England was the most valuable of his kingdoms and he soon returned, leaving Denmark to be ruled by a regent.