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===Battle of Svolder===
[[File:Norway 1000 AD.png|thumb|right|upright|Division of Norway after the [[Battle of Svolder]] according to ''[[Heimskringla]]'': The red area was under direct Danish control, with Swein ruling it as a Danish extension.
[[Eiríkr Hákonarson]] ruled the purple area as a fiefdom from Swein Forkbeard. The yellow area was under [[Sveinn Hákonarson]], his half-brother, held as a fief of [[Olof Skötkonung]], the Swedish king.]]

[[Harald Bluetooth]] had already established a foothold in Norway, controlling [[Viken (region)|Viken]] in {{Circa|970}}. He may have lost control over his Norwegian claims following his defeat against a German army in 974.

Swein built an alliance with Swedish king [[Olof Skötkonung]] and [[Eirik Hákonarson]], [[Jarl of Lade]], against Norwegian king [[Olaf Tryggvason]].
The [[Kings' sagas]] ascribe the causes of the alliance to Olaf Tryggvason's ill-fated marriage proposal to [[Sigrid the Haughty]] and his problematic marriage to [[Tyra of Denmark|Thyri]], sister of Swein Forkbeard. According to the sagas, Sigrid pushed Swein into war with Olaf because Olaf had slapped her.<ref>{{Cite book|last=Bagge|first=Sverre|url=https://books.google.com/books?id=NFJNAgAAQBAJ|title=Cross and Scepter: The Rise of the Scandinavian Kingdoms from the Vikings to the Reformation|date=2014|publisher=Princeton University Press|isbn=978-1400850105|pages=31|access-date=22 July 2021|archive-date=23 April 2023|archive-url=https://web.archive.org/web/20230423120140/https://books.google.com/books?id=NFJNAgAAQBAJ|url-status=live}}</ref>

The allies attacked and defeated king Olaf in the western [[Baltic Sea]] when he was sailing home from an expedition, in the [[Battle of Svolder]], fought in September 999 or 1000. The victors divided Norway among them. According to the account of the ''Heimskringla'', Swein regained direct control of [[Viken (region)|Viken district]].

King [[Olof Skötkonung|Olof]] of Sweden received four districts in [[Trondheim]] as well as [[Møre og Romsdal|Møre]], [[Romsdal]] and [[Rånrike]] (the ''[[Fagrskinna]]'', by contrast, says that the Swedish part consisted of [[Oppland]] and a part of Trondheim). He gave these to his son in law, Jarl [[Svein Hákonarson]], to hold as a vassal. The rest of Norway was ruled by Eirik Hákonarson as King Svein's vassal.

The Jarls Eirik and Svein proved strong, competent rulers, and their reign was prosperous. Most sources say that they adopted Christianity but allowed the people religious freedom, leading to a backlash against Christianity which undid much of [[Olaf Tryggvason]]'s missionary work.<ref>This is according to ''Heimskringla'' and ''Fagrskinna'', see Lee M. Hollander (trans.) (1991) ''Heimskringla'', p. 244 and Finlay Finlay, Alison (editor and translator) (2004) ''Fagrskinna'', p. 130.

According to ''[[Historia Norwegie]]'' and ''[[Ágrip]]'', the Jarls actively worked to uproot Christianity in Norway, see Driscoll, M.J. (editor) (1995). Ágrip af Nóregskonungasǫgum. Viking Society for Northern Research, p. 35 and Ekrem, Inger (editor), Lars Boje Mortensen (editor) and Peter Fisher (translator) (2003). Historia Norwegie (2003), p. 101.</ref>