The underlying cause of this conflict was Olaf Tryggvason' s refusal to recognize the authority of Denmark over Norway, which deprived King Svein Forkbeard of Denmark his fair share of Norway. A natural ally of the king of Denmark of course was Jarl Eirik Hakonarson who was the son of Jarl Hakon who was killed by King Olaf. Jarl Eirik, of course, wanted his father' s lands back, and wanted the old age rite of revenge against his father' s killer.
As with all histories the victors wrote the stories that have come down to us. Adam of Bremen, in the late, 1000's , (app. 1080), wrote a history of the battle based on what King Svein of Denmark related to him. Subsequent versions of the battle can be found written by Saxo Grammaticus in his epic work on the Danes Gesta Danorum, which was written in about 1200 A.C.E. These two rendition of the battle tend to leave out the role of the Norwegians under Jarl Eirik and give all the credit to the Danes. On the other hand in Snorri Sturluson' s Heimskringla Jarl Eirik's role is played up and he is the one who attacks the Long Serpent and forces King Olaf to jump overboard. History is all about perspective and national pride my friend.
An odd side story to this battle is that Olaf Tryggvason was married to Thyri the sister of Svein Forkbeard. According to several accounts Thyri begged her husband to attack Svein Forkbeard and his Danes because her brother had refused to give her the dowry that he had promised her. On the other side of the coin Svein Forkbeard' s wife Sigrid, known to many as Sigrid the Haughty, hated Olaf Tryggvason because he struck her for refusing his proposal of marriage. She then convinced her husband King Svein to make war upon the Norwegian king. King Svein then made the alliance with King Olaf of Sweden and Jarl Eirik of Norway.
The exact location of the battle is also something that I will not argue here. There are three or four different opinions out there of where the battle was actually fought. The most prevalent of these being by an island called Svolder, near the German island of Rugen, or near Oresund. Also the number of ships vary from source to source. The numbers for the Danish / Swedish / Norwegian alliance are from 70 to well over 100. All accounts say that Olaf Tryggvason was greatly outnumbered. The Sagas state that some of this imbalance was caused by Jarl Sigvaldi deserting King Olaf, and joining the Danish fleet. I guess when he saw the odds against him he decided to side with the stronger force.
End Part 1
Go with Odin' s wisdom, Freyja' s love, and Thor's protection !