Battle of Söder Strait

The Battle of Söder Strait was a naval battle during King Eerik's War fought between the Kingdom of Karelia and an alliance of Norse Jarldoms off the coast of the island of Söder on June 18, 599. The Karelian fleet, despite being personally led by King Eerik, was decisively defeated by the Norse alliance and forced to withdraw. The Norse victory allowed them to begin raiding Karelian territory in retaliation, turning the tide of the war, eventually leading to the Serkir Shahdom entering on the side of the Norse, and the siege of the then-capital of Karelia, Valjala. The battle also demonstrated the weakness of Bolenesian fire, and the danger it posed when used improperly.

Background
The paranoid Grand Duke Eerik I Karjalainen declared himself the King of Karelia in 596, launching an invasion of Norse territory the following year. Eerik's initial campaign achieved some successes, defeating the fleets of some Norse Jarldoms and pillaging several outposts. After the first year of fighting, most of the Norse Jarls formed an alliance to counter the Karelians. Even with their combined fleets, the Norse found difficulty in countering the Karelians' use of Bolenesian fire, a liquid flammable substance spewed from the bows of specially designed galleys and extremely effective in destroying enemy vessels.

In summer of 599, King Eerik gathered a fleet of 23 galleys in preparation for a planned invasion of Skjálfandi Bay. Norse spies discovered this, and relayed the information to the alliance of Jarls. Seeing an opportunity, and wishing to achieve a major victory against the Karelians, the alliance dispatched a fleet of galleys and longboats commanded by Ivar the Hard-headed to intercept King Eerik. On route to his target, Eerik gave order to stop and pillage the island of Söder, aiming to gather a surplus of supplies for their voyage and intended siege of Skjálfandi Bay.

Battle
Late in the evening of June 17, the Norse fleet became aware of Karelian activities on the island, maneuvering their fleet towards the island, arriving early in the morning. They were detected by the Karelian sailors, who promptly assembled their fleet. By the orders of King Eerik, the Karelian fleet took a defensive position on the southwestern send of the island strait, with the fleet's three fire ships positioned in the front of the battle line.

The Norse, in response, launched a frontal assault through the strait, but also sent a detachment of 11 longships around the smaller island of Södermalm. This flanking move was met with some success, and the crews of the longships were able to seize two Karelian galleys. However, on the whole, the Karelians' Bolenesian fire was proving too much for the Norse.

The tide of the battle turned when a Norse galley, set aflame by Bolenesian fire, successfully rammed itself into a Karelian galley in a suicide attack. The fire spread to the Karelian galley, which then subsequently extended to several other Karelian vessels. This caused mass panic and confusion among the Karelians, many of whom believed it to be an incident of friendly fire. A rout began, and the Norse pursued, sinking another galley, before ending the chase. By the end of the battle, two Karelian ships were captured, five were sunk, and three were destroyed in the fire.