The Chronicle of The Kings of Norway
by Snorri Sturlson | c.1179-1241 | 320,198 words
The "Heimskringla" of Snorri Sturlason is a collection of sagas concerning the various rulers of Norway, from about A.D. 850 to the year A.D. 1177....
The Emperor Otta (Otto) was at that time in the Saxon country, and sent a message to King Harald, the Danish king, that he must take on the true faith and be baptized, he and all his people whom he ruled;
says the emperor,
"we will march against him with an army."
The Danish king ordered the land defence to be fitted out, Danavirke  (the Danish wall) to be well fortified, and his ships of war rigged out. He sent a message also to Earl Hakon in Norway to come to him early in spring, and with as many men as he could possibly raise.
In spring (A.D. 975) Earl Hakon levied an army over the whole country which was very numerous, and with it he sailed to meet the Danish king. The king received him in the most honourable manner. Many other chiefs also joined the Danish king with their men, so that he had gathered a very large army.
Footnotes and references:
Danavirke. The Danish work was a wall of earth, stones, and wood, with a deep ditch in front, and a castle at every hundred fathoms, between the rivers Eider and Slien, constructed by Harald Blatand (Bluetooth) to oppose the progress of Charlemagne. Some traces of it still exist. — L.