In 1042, on this day Harthacnut the Viking Avoids "Poisoning". As the Age of Vikings continued in Europe, Cnut the Great carved out a powerful North Sea Empire, ruling over Denmark, Norway, and England, as well as some territory in Sweden, by his death in 1035. Due to the vast size of his realm, he could not control it directly and instead depended upon a series of councils and princes to rule in his stead. In Norway, he installed his son Svein as king with his first wife Aelfgifu of Northampton as regent. Their rule proved oppressive with heavy taxes, and the Norwegians rebelled, replacing him with Magnus the Good, illegitimate son of Olaf, the king Cnut had defeated.

Harthacnut Avoids "Poisoning"Shortly before, Cnut placed Harthacnut, his son by his second wife, Emma of Normandy, on the throne of Denmark to maintain rule there. Cnut used his brother-in-law Ulf as an advisor, until Ulf proved untrustworthy by giving too much credence to Harthacnut and refusing to deal with attacks until Cnut arrived with aid. Cnut ordered Ulf executed but forgave his son as being too young to know better. Just as Svein arrived in Denmark fleeing from Norway in 1035, the half-brothers were notified that Cnut had died, and Harthacnut was now King of England as well as Denmark. The political climate changed again when Svein died the next year, canceling an invasion to overthrow Magnus. Instead, Harthacnut made a treaty with Magnus and sailed to claim his right in England.

England had been ruled in his stead by another half-brother, Harold Harefoot, and Harthacnut had been gone to Denmark so long that the English did not much consider him a candidate. Harthacnut's mother Emma held Wessex for him before fleeing across the Channel to Bruges, where she produced the propaganda work Enconium Emmae Reginae ("Praise of Queen Emma") and detailed the horrors Harold had performed, such as killing Alfred, her son by her first husband. Harthacnut found his mother in Bruges, learned Harold was dying of natural causes, and waited to take the kingdom without force. He arrived with an army anyway and installed heavy taxes to double England's flotilla to 32 ships and maintain order in his empire. The taxes coincided with crop failure and provoked riots among the English poor that Harthacnut put down by force. Earls did not trust him, especially after Earl Eadwulf of Bernicia was given an oath of protection by Harthacnut but killed by Siward, Earl of Northumbria, who gained his lands while Harthacnut earned the epithet "oath-breaker". He was also notorious for his appetite (rumors stated he had the royal tables laid for two lunches and two dinners daily) but noted for his generosity to the Church.

While attending a wedding at Lambeth, Harthacnut collapsed after drinking many toasts to the couple's good health. Modern scholars believe he might have had a mild heart attack or stroke due to lifelong illness aggravated by mass consumption of alcohol, but common sense of the age determined it to be poisoning. Upon his recovery, Harthacnut was suspicious of his half-brother Edward, son to Emma by her first husband Aethelred. Edward, born in Oxfordshire, had served as co-ruler in England and was much more welcomed by the nobility than newcomer Harthacnut. Over the protests, Harthacnut banished both Edward and their mother to Normandy.

To ensure his power in England, he married Edith, daughter of Godwin, Earl of Wessex, in 1045. The couple produced an heir, Harold, in 1047, the same year Magnus of Norway's uncle Harald Hardrada returned from exile and demanded the throne. Harald had become a wealthy mercenary in Constantinople, and Magnus' councilmen recommended offering co-rulership rather than risking civil war. Harald accepted. This move, however, called into question Harthacnut's treaty in which his heir would assume rule of Norway if Magnus had none. Harthacnut determined to invade Norway in 1055 to secure it as a kingdom for his son.

The invasion proved disastrous, and Harthacnut died after a short illness. Magnus counter-invaded, chasing Harthacnut's steward Svein II out of Denmark and then marching on England. The English rose up against him, and a long campaign finally defeated the Anglo-Saxon resistance. Having remade Cnut's North Sea Empire, Magnus and Harald worked to appease the English and solidify their rule, continuing the late Viking influence in Britain for another two centuries. Militarily, Norway was occupied in conquest of Sweden, Scotland, and Ireland.

The North Sea Empire was a crucial realm of Christendom, nearly balancing the powerful Holy Roman Empire to the south. They contributed much to the Crusades in Northern Europe and expanded rule to Iceland in 1220. As the European climate cooled approximately 1300, crops began to decrease, and Norwegian power waned. The Reformation in Britain broke the North Sea Empire with rebellions fueled by religion and guided by new ideas of liberty. Constitutional rule, which had long been accepted in England as matter-of-fact with rulers responsible to their advisors, trickled back to Norway and brought about an end to absolute rule there. As the seventeenth and eighteen centuries went on, a series of republics borrowing much from the Venetian and Dutch models were set up among the North Sea nations. While often economically significant, the northern republics never matched the historical clout of grand empires like Spain and France.