September (9,10,11)?*

The Battle of Svolder was a naval battle fought in September 999 or 1000 in the western Baltic Sea between King Olaf Tryggvason of Norway and an alliance of his enemies. The backdrop of the battle was the unification of Norway into a single state, long-standing Danish efforts to gain control of the country, and the spread of Christianity in Scandinavia.

King Olaf was sailing home after an expedition to Pomerania, when he was ambushed by an alliance of Svein Forkbeard, King of Denmark, Olof Skötkonung (also known as Olaf Eiríksson), King of Sweden, and Eirik Hákonarson, Jarl of Lade. Olaf had only 11 warships in the battle against a fleet of up to 70. His ships were captured one by one, last of all the Ormen Lange, which Jarl Eirik captured as Olaf threw himself into the sea. After the battle, Norway was ruled by the Jarls of Lade as a fief of Denmark and Sweden.

* There is considerable conjecture about the actual date.