The Kalundborg War was a conflict spanning three years between Sweden, Norway and Holstein, and an alliance consisting of Denmark, Lübeck, Hamburg, Rostock, Wismar, Greifswald, and Stralsund.

[1] In 1341, Valdemar IV attacked Holstein and Duchess Ingeborg, Magnus's mother, who at this time resided at Kalundborg Castle in Zealand.

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.

[2][3][4] 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.

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.