King Cnut of England and Denmark seeks to restore Danish rights in Norway, lost in 1016 upon the ascension of native Olaf Haraldsson as monarch.

Olaf and King Anund Jacob of Sweden, seeing the combined Anglo-Danish kingdom as a threat—Cnut's father Sweyn had asserted power over both their countries—take advantage of Cnut's being in England to attack the Danes in the Baltic Sea in 1025 or 1026, and are joined by Ulf Jarl, Cnut's Danish regent, and his brother.

Ulf Jarl is the son of Thorgils Sprakalägg, who is claimed to have been the son of Styrbjörn the Strong, a scion of the Swedish royal house, by Tyra, the daughter of king Harald Bluetooth of Denmark.

However, Thorgils' parentage may have been invented to glorify the royal dynasty founded by Ulf's son, Sweyn Estridson.

Ulf had joined Cnut’s expedition to England.

He had married Cnut's sister Estrid in about 1015 and was appointed the Jarl of Denmark, which he rules when Cnut is absent.

He is also the foster-father of Cnut's son Harthacnut.

When the Swedish and Norwegian kings attack Denmark, Ulf persuades the freemen, who are discontent at Cnut's absentee rule, to elect Harthacnut king.

This is a ruse on Ulf's part, as his role as Harthacnut's guardian will make him the ruler of Denmark.

When Cnut learns what has happened, he returns to Denmark and confronts his enemies at the Battle of the Helgeå, where the Swedish and the Norwegian navies led by kings Anund Jacob and Olaf II lie in wait up a river for the navy of King Cnut, which is commanded by Ulf Jarl.

Cnut's navy is massive and his own ship is said to have been eighty meters long.

The Swedish and the Norwegian kings had ordered a large dam made of peat and lumber to built on the river.

When the Danish navy sails in, the water is released and a great many Danes and Englishmen drown in the deluge.

However, Cnut's men are apparently able to win the battle.

The outcome is disputed, but Cnut comes out best; Olaf flees and the threat to Denmark is dispelled.

The apparent victory leaves Cnut as the dominant leader in Scandinavia.

The battle is retold in skaldic poetry and in sources such as the Danish Gesta Danorum by Saxo Grammaticus and the Icelandic Saga of Olaf the Holy by Snorri Sturluson.

Opinions are divided on whether the location was at Helgeå in Uppland or the Helgeå of eastern Skåne.

In the Anglo-Saxon Chronicle, the battle is dated to 1025 and the Swedes won the battle.