Cnut was born into the Danish royal house, though no record confirms his exact birth date or place. His father Swein Forkbeard ruled Denmark as a son of Harald Bluetooth and heir to Harthacnut I. The mother remains disputed across sources. The Chronicon of Thietmar of Merseburg names her Świętosława, daughter of Mieszko I of Poland. Norse sagas like Heimskringla by Snorri Sturluson call her Gunhild, daughter of Burislav, king of Vindland. Adam of Bremen in Gesta Hammaburgensis ecclesiae pontificum uniquely equates Cnut's mother with the former queen of Sweden, wife of Eric the Victorious. This creates confusion about whether Swein had one wife or two. Some historians believe Adam made an error, suggesting Swein married first Gunhild, then Sigrid the Haughty after Gunhild died. Flateyjarbók offers a glimpse of Cnut's youth. It states he learned soldiery from chieftain Thorkell the Tall at their stronghold on Wollin island off Pomerania. A skaldic verse called Knútsdrápa mentions Cnut was "of no great age" when he first went to war. If he accompanied Swein's 1003 invasion of England, his birth might be near 980 or 990. If he joined the 1013 campaign instead, it could suggest 1000 as his birth year.
Conquest Of England
In summer 1015, Cnut's fleet set sail for England with perhaps 10,000 men aboard 200 longships. The invading army consisted mostly of mercenaries drawn from across Scandinavia. They fought fourteen months of close and grisly warfare against Edmund Ironside, eldest son of King Ethelred. Early in September 1015, Cnut landed in Sandwich and sailed around Kent to Wessex. He harried Dorset, Wiltshire, and Somerset with intensity unseen since Alfred the Great's days. Eadric Streona, Ealdorman of Mercia, deserted Ethelred with forty ships and crews to join Cnut. Thorkell the Tall also defected, bringing Jomsviking forces into service. In early 1016, Vikings crossed the Thames and ravaged Warwickshire. Edmund Ironside tried to oppose them but failed repeatedly. The English army disbanded when London citizens refused to fight alongside their king. Cnut devastated eastern Mercia mid-winter. Uhtred Earl of Northumbria joined Edmund northward to harry Staffordshire, Shropshire, and Cheshire. Cnut sent Thurbrand the Hold to massacre Uhtred and his retinue. Eiríkr Hákonarson arrived with another force to support Cnut at this point. Prince Edmund remained in London after Ethelred died on the 23rd of April 1016. He was elected king but could not hold the city. On the 18th of October 1016, Danes fought Edmund's army near Ashingdon or Ashdon in Essex during the Battle of Assandun. Eadric Streona withdrew his forces from the fray, causing decisive defeat for Edmund. Cnut pursued him into Gloucestershire where another battle likely occurred near Forest of Dean. An island near Deerhurst hosted peace negotiations between wounded kings. All England north of the Thames went to Cnut while south stayed with Edmund including London. Accession to entire realm passed to Cnut upon Edmund's death on the 30th of November 1016. West Saxons accepted Cnut as king of all England crowned by Lyfing Archbishop of Canterbury in London 1017.