The Battle for Vestfold is a decisive military engagement that is part of the Rus Invasion of Scandinavia. It depicts the confrontation between the Kingdoms of Norway and Kattegat and the Kievan Rus. The event takes place in Season 6 Episodes 10 and 11, The Best Laid Plans and King of Kings.
Following his defeat to his brothers during the Siege of Kattegat, Ivar fled Norway. Traveling along the Silk Road, Ivar is captured and brought to Kiev. Ivar starts developing a bond with Prince Oleg, whose ambitions far outstretch that of the Kingdoms of Rus. Having killed one of his brothers and mutilated the other one, as well as capturing the heir to the Rus throne, Prince Oleg starts seeking the opportunity to invade Scandinavia with the help of Ivar, plotting to reinstate him as a puppet king of Kattegat. Oleg orders a vast army of some 13,000 warriors to gather in Kiev. His force is not only large, but disciplined, well-armed, provisioned, and is made up of seasoned and skilled fighting men. After deploying his large troops in front of the walls of his capital, Prince Oleg states that his invasion's goal is to rid Scandinavia of paganism, instead spreading Christianity, as the Rus, unlike their Viking counterparts, were earlier Christianized.
While Ivar travelled to Kiev, Bjorn consolidated his power as the new King of Kattegat. However, Bjorn was tricked into helping King Harald who had been attacked by King Olaf and subsequently captured. King Olaf ambushed Bjorn and a few hundred of his men during a night attempt to free King Harald. A few days later, King Olaf reveals to Bjorn his intentions to gather all Norse jarls and kings in order to elect an overall leader. During the election, King Harald was surprisingly elected, possibly having corrupted many jarls, including Jarl Thorkell. Bjorn narrowly avoided murder by KIng Harald's men after being saved by the outlaw Erik the Red.
Returning to Kattegat, Bjorn was confirmed as king by his subjects. He also learnt through Ubbe that Ivar had joined the Kievan Rus. Prince Oleg had sent a small vanguard force under his general Ganbataar and Ivar to scout Norse lands. During one of these attacks, Hvitserk, having killed Lagertha while hallucinating Ivar, rejoined his brother. Another bloody attack on a coastal village named Istrehagan in King Harald's kingdom informed the latter about the Rus threat. King Harald decided to mobilize his forces to meet this new threat and called on his ancient ally Bjorn to join him. Bjorn reluctlantly agreed.
Most of the newly sworn vassals of King Harald failed to answer his summons, with the notable exception being the powerful Jarl Thorkell. The remaining forces had to be drawn from the already decimated forces of Kattegat, as well as the spent armies of King Harald.
The Viking Army, made up of contingents drawn from other earldoms, the former Kingdom of Vestfold-Rogaland, the earldom of Jarl Thorkell, the annexed Kingdom of Ringariki, and the Kingdom of Kattegat, formed up on a vast beach in Vestfold. The troops gathered probably did not exceed 2,000 warriors. Another force was dissimulated at the mouth of a river in order to prevent Rus ships from sailing upriver and catching the Viking force on their rear. The Rus onslaught started when barges packed with soldiers reached the beach. They were met by volleys of arrows by Kattegat archers under Bjorn and Erik's command. Contingents of well-armed Rus soldiers started pouring onto the beach, as well as cavalrymen, who started dislocating hastily erected barricades with grappling hooks protecting the Viking forces. Soon, a vicious close quarter combat emerged which gradually turned into the Rus' favor as they were more numerous.
Meanwhile, a force of Rus ships led by Prince Oleg and Ivar had moved to the mouth of the river and they were welcomed by Norwegian archers. The Rus replied with volleys of arrows and forced through a barricade, knocking over Viking warriors and engaging armed men in their longships. However, this second Viking force couldn't prevent the Rus ships sailing through the river's mouth, until they reached a small mountain. Prince Oleg's forces climbed the mountain and hundreds of Rus soldiers made for the Vestfold capital at the rear of the Viking forces.
On the beach, the Rus onslaught proved too much for the smaller Viking force who were being pushed back with heavy casulaties by the mass of Rus soldiers. Jarl Thorkell, though fighting bravely, was slain. The remaining Viking archers under Erik sent huge piked boulders down the hill which went smashing into the advancing Rus forces. King Harald was also felled by Rus soldiers. Another Rus force, which had climbed the mountain, then emerged at the rear of the Viking forces in order to complete their rout. Many remaining Viking warriors either fled or fought and died on the beaches, culminating with Bjorn being stabbed by Ivar, mortally wounding him.
The battle was a decisive and crushing victory for the Kievan Rus. Thanks to their numbers and to Ivar's strategic brilliance, the Rus onslaught took the Vikings in the front and in the rear. The fall of both King Harald and King Bjorn, both seemingly dead or at least seriously injured, also deprived the remaining Viking forces of their leaders. The Norse troops were scattered and severely weakened, thus paving the way for further Rus advance into Scandinavia.