Kalundborg War

The Kalundborg War was a conflict spanning three years between Sweden and Norway, allied with Holstein, and an alliance consisting of Denmark, Lübeck, Hamburg, Rostock, Wismar, Greifswald, and Stralsund. Although Denmark achieved some victories, the war ended favourably for Sweden and Holstein.

Prelude
The exact details surrounding this war are quite unclear. The conflict likely arose due to Valdemar IV's ambitions to revive Denmark's past glory after he became king in 1340. When he assumed the throne, the nation was largely under the control of foreign rulers.

1341
In 1341, Valdemar IV attacked Holstein and Duchess Ingeborg, Magnus's mother, who at this time resided at Kalundborg Castle in Zealand. Valdemar was supported by the Hanseatic cities of Lubeck, Hamburg, Rostock, Wismar, Stralsund and Greifswald, who likely sided with Valdemar to restore the power balance in the north.

The balance of power in the north had been seriously disrupted by the weakness of Denmark and the strength of Magnus's Swedish-Norwegian union. On the Danish front, the Holsteiners apparently succeed in rescuing Kalundborg and the Danes had to lift the siege. A truce was concluded around 22 September between Valdemar, Magnus and Ingeborg. It's likely that Swedish forces did not participate at all or to a very small extent in the battles on during the year, but a truce was nevertheless concluded between Valdemar and Magnus on 25 November 1341.

1342
In 1342, the fighting began again, this time with active Swedish participation. On June 26, a battle was fought outside Copenhagen between Swedes and Holsteiners and Danes and their Hanseatic allies on the other. The battle ended with a Danish allied victory, 350 Swedes are said to have died in the fighting and many were taken as prisoners to Lubeck. At the time of the battle, Copenhagen Castle was apparently in Swedish hands, according to one source, Magnus won the castle by paying 7,000 marks of silver to a knight named von Plessen.

According to other sources, Copenhagen in its entirety was occupied by Magnus. The further development of the war are also shrouded in obscurity, Magnus prepared new forces but in October the Hanseatic cities declared that they were willing to make peace and let the disputed issues go to arbitration. This led to an unsuccessful meeting between Magnus and representatives of the Hanseatic cities on 6 December 1342.

1343 and peace
On 17 July, probably through the mediation of Albrecht of Mecklenburg, peace was finally concluded between Sweden and the Hanseatic cities. When the Hanseatics pulled out, Valdemar IV's situation was worrying, he faced a two-front war with Sweden in the north and Holstein in the south. On November 18, he was forced to make peace on terms favorable to Sweden. Valdemar acknowledged the sale of Scania, Blekinge and Halland from the years 1332 and 1341. He also acknowledged that the full purchase price for the sale has been paid and waives the right of repurchase that previously existed in the purchase. Magnus also renounced all claims west of the Sound. A bond of friendship was also signed between Valdemar and Magnus on the same day.