His long and eventful reign was marked by Danish raids on the Carolingian Empire of Louis the Pious, son and successor of Charlemagne.

Horik's father was King Gudfred, known for his successful raids and wars against Charlemagne's Frankish empire and against the Obodrites.

The five brothers found support from all over the Danish realm, defeated Harald and Ragnfred with relative ease, and expelled them from Denmark.

The four brother kings had gathered a fleet of 200 ships and were posted on the island, refusing to offer the imperial troops battle.

Next, the Obodrite ruler Slavomir, a vassal of Louis, defected since the emperor forced him to share power with another prince, Keadrag.

In a dramatic twist of alliances the remaining king-brothers, one of which was obviously Horik, asked Harald Klak to share power with them.

However, the sons of Gudfred irritated the emperor by allying with the Obodrite prince Keadrag in a "faithless" way, causing Louis to reinstate Slavomir in his stead.

Exasperated, Horik and his brother gathered an army, crossed the Eider and attacked the imperial troops in the neighbourhood.

According to the Vita Ansgari the king trusted Ansgar to the degree that the latter was allowed to follow the meetings with his privy council.

When Ansgar planned a missionary expedition to the Swedes in about 852, Horik helped prepare the trip, providing the archbishop with his personal envoy and a message to the Swedish King Olof.

[14] The king (or his partisans) had found refuge in Sweden in his youth and may have entertained good relations with part of the Swedish elite.

In spite of his flirtation with the Christian mission towards the end of his life, his reign was filled with Viking activities directed against the Frankish kingdoms, often but not always approved by himself.

In 836, Horik sent an emissary to Emperor Louis declaring that he had nothing to do with the raids on Frisia, and two years later he assured that he had executed those responsible.

[15] In return he asked the emperor to cede the lands of the Frisians and Obodrites, which Louis found highly offensive.

The expelled Swedish King Anund secured assistance from the Danes in the 840s in order to regain his kingdom, presumably with Horik's approval.

[17] After the break-up of the Frankish Empire in 843, Horik temporarily broke off his old conciliatory stance and began open hostilities towards the West and East Franks.

In 845 a fleet under one of Horik's chiefs, Ragnar, sailed up the Seine and attacked Paris in the West Frankish Kingdom.

Paris was sacked and King Charles the Bald was forced to flee, eventually paying Ragnar a sum of 7,000 pounds of silver as ransom.

A considerable booty was brought back to Denmark, though many Vikings including Ragnar were later claimed by Frankish sources to have perished in a violent illness.

[18] Still, Frisia was ravaged once again in 846, and in the following year the three Frankish monarchs and brothers, Charles, Louis and Lothair I, agreed to intimidate Horik in order to gain peace.

[19] It has been argued that the Viking raids in West Europe weakened rather than strengthened Horik's authority, as he was increasingly unable to control the warrior chiefs.

[20] In 850, Horik I and two unnamed nephews (called "nepotes") partitioned the kingdom into thirds, ending the unified kingship.

[24] Horik II disappeared some time between 864 and 873 and was the last known ruler of the Sigfredian dynasty, as Denmark seems to have lost its political cohesion in the late 9th century.