The Kattegat Civil War is a blood feud and war of succesion within the kingdom of Kattegat. The war was presumably won by the forces of Ivar the Boneless and King Harald of Vestfold-Rogaland, who won a sounding victory at the second battle after suffering a high casuatie setback at the first engagement.
The Wives of Ragnar
While Ragnar was an earl, he had commited adultery with a Geatish princess named Aslaug and sired Ubbe. Wishing to have many sons and being smitten with the princess, Ragnar suggested to his then wife Lagertha that they arrange for a polygamous marriage. Ragnar wanted to have both women in his life. Lagertha was offended by this suggestion and divorced Ragnar. She would later remarry to the earl Sigvard and later become an earl herself. Aslaug would eventually give Ragnar the sons Hvitserk, Sigurd and Ivar.
Ragnar eventually became king and following his Frankish campaigns, Kattegat became a major trading centre in Norway. During Ragnar's exile, Lagertha had begun to harbour animosity towards queen Aslaug and planned to conquer Kattegat. When Bjorn left with the majority of the fighting men and women in Kattegat for his Mediterranean Raid, Lagertha launched a swift and brutal takeover of Kattegat. Aslaug surrendered and promised to tell her sons not to take vengence. As Anslaug left, Lagertha shot an arrow into her back.
The feelings among the brothers about the murder of Aslaug was split. Bjorn did not approve of his mother's actions but would not allow his younger brothers to raise a hand against her. Sigurd loathed his mother and Hvitserk had mixed feelings about her. Ivar, who was the closest to Aslaug, demanded vengence and Ubbe shared his brother's feelings.
Sigurd held a long-running feud with his youngest brother, Ivar. This was in part due to a clash of personalities, and in part jealousy, as due to Ivar's condition their mother often favoured him over Sigurd. In a celebratory feast in the former Wessex royal villa, Bjorn as leader of the Army gave several toasts to victory and the destruction of two English kingdoms. Whilst discussing the way forward, he and Ivar got into a heated argument, and exchanged many insults and curses. In a rage, Ivar threw an axe at Sigurd, which lodged in his left abdomen. He stumbled slightly, in an attempt to retaliate, but ultimately fell and died. His death created a definite rift among the sons of Ragnar Lothbrok.
Four sons and one crown
In Nordic society, all the sons of the previous king could be considered proper candidates to inherit the crown. Bjorn, being the only living child of Lagertha, and the oldest son of Ragnar would have a strong claim. Ivar's claim as king is supported by King Harald and his older brother Hvitserk.
The Gathering Storm
As Harald has ambitions to annex Kattegat, Ivar and Hvitserk sought an alliance with him after defeating the Saxon's at York and establish a stong base there. Ivar promised to make Harald his heir in exchange for his military aid in overthrowing and killing Lagertha. Harald's queen, Astrid, was a former lover of Lagertha and she sent a warning through Hakon to Kattegat.
- The war losely resembles the conflicts between Eirik Bloodaxe and his brothers in the 10th century, where the many sons of King Harald did not accept his nomination of Eirik as the high king of Norway.