In the Viking period Denmark was bigger than it is today. In Jutland the border was located further to the south at the Eider River, and was marked by the defensive system known as the Danevirke. The trading settlement of Hedeby was located close to the border.
In the east the border was not marked by Øresund, like it is today. Østfold, which is located in the south of present-day Norway, was probably part of Denmark in the 890s and perhaps before this. Norway was under Danish rule for long periods between 970 and 1035.
The North Sea Empire
The throne of England was also occupied by Danish kings in the Viking period. Under Sweyn Forkbeard the raids against England increased, and in 1013 Sweyn became the first Viking king to conquer all of England. He was recognised as England’s new king, but died the year after.
Sweyn Forkbeard laid the foundations of a Danish-Norwegian-English North Sea Empire, which his son Canute the Great later enlarged and cemented. In a letter to the English people in 1027 Canute the Great called himself “Canute, king of all of England and Denmark and the Norwegians and part of Sweden.”
After Canute’s death in 1035, two of his sons succeeded him on the English throne for a short period until 1042, but subsequently the North Sea Empire collapsed.
Based upon, amongst other things, the archaeological evidence, it is believed that the population of the area now occupied by Denmark was around 500,000 in the year 800 AD. This steadily increased until the 1200s, when the population reached c. 1 million.