Edmund's reign was marred by a war he had inherited from his father; his cognomen "Ironside" was given to him "because of his valour" in resisting the Danish invasion led by Cnut.

[2] In summer of 1013 Sweyn Forkbeard launched a full-scale invasion of England, driving out Aethelred by the end of the year.

After regaining the throne, the royal family set about strengthening its hold on the country with the assistance of Eadric Streona (Edmund's brother-in-law).

Sigeferth's widow Ealdgyth was imprisoned within a monastery, but in summer of 1015 Edmund unofficially named himself the Earl of the East Midlands and raised a revolt against his father.

Without the king's permission he took Ealdgyth from the monastery, and married her; it would have been a politically advantageous marriage, since she was a member of one of the strongest families in the Midlands.

His mother died around 1000,[3] after which his father remarried, this time to Emma of Normandy, who had two sons, Edward the Confessor and Alfred and a daughter Goda.

[4] The Life of Edward the Confessor, written fifty years later, claimed that when Emma was pregnant with him, all Englishmen promised that if the child was a boy they would accept him as king.

[2] When Sweyn Forkbeard seized the throne at the end of 1013 and Æthelred fled to Normandy, the brothers do not appear to have followed him but stayed in England.

In late 1015 Edmund raised an army, possibly assisted by his wife's and mother's links with the midlands and the north, but the Mercians under Eadric Streona joined the West Saxons in submitting to Cnut.

[2] Æthelred died on 23 April 1016, and the citizens and councillors in London chose Edmund as king and probably crowned him, while the rest of the Witan, meeting at Southampton, elected Cnut.

They renewed the siege while Edmund went to Wessex to raise further troops, returning to again relieve London, defeat the Danes at Otford, and pursue Cnut into Kent.

Contemporary accounts do not suggest that he was murdered, but soon after the Norman Conquest Adam of Bremen wrote that he had been poisoned, and twelfth century writers stated that he was stabbed or shot with an arrow while sitting on a toilet.

In Lawson's view, the intensity of Edmund's struggle against the Danes in 1016 is only matched by Alfred the Great's in 871, and contrasts with Æthelred's failure.