Hamburg had been elevated to an archbishopric…

Hamburg had been elevated to an archbishopric by Pope Gregory IV in 831 on the initiative of Louis the Pious in order to oversee the Saxon territory and to support the introduction of Christianity to Scandinavia.

Hamburg had been united with Bremen two years later as the bishopric of Hamburg-Bremen.

The first bishop, Ansgar, will become known as the Apostle of the North.

Horik has refused to convert to Christianity, as it is his enemies' religion, and resists attempts by Bishop Ansgar to proselytize the Danes.

Six hundred Viking ships sail up the River Elbe in 845, sack Hamburg, at this time a town of around five hundred inhabitants, and destroy St. Mary's Cathedral.

The East Frankish king Louis the German in response sends a diplomatic mission, headed by Count Cobbo (one of two court counts), to the court of Horik, demanding of the Danish king that he submit to Frankish overlordship and pay reparations for the invasion.

Horik eventually agrees to the terms and requests a peace treaty with Louis, while also promising to turn back the treasure and captives from the raid.

Horik most likely wants to secure the border with Saxony as he faces a conflict with King Olof of Sweden and domestic struggles.

By the treaty, Louis demands Horik's obedience, which is further secured by Horik regularly sending embassies and gifts to Louis, and his suspension of support to Viking raiders.