Æthelred makes an alliance with Normandy in 1001, through marriage to the Duke's daughter Emma, in the hope of strengthening England.

He now makes a great error: in 1002 he orders the massacre of all the Danes in England, which has serious consequences.

It angers Sweyn, who unleashes a decade of devastating attacks on England.

Northern England, with its sizable Danish population, sides with Sweyn.

By 1013, London, Oxford, and Winchester have fallen to the Danes.

Æthelred flees to Normandy and Sweyn seizes the throne.

Sweyn suddenly dies in 1014, and Æthelred returns to England, confronted by Sweyn's successor, Cnut.

However, in 1016, Æthelred also suddenly dies.

Cnut swiftly defeats the remaining Saxons, killing Æthelred's son Edmund in the process.

Cnut seizes the throne, crowning himself King of England.

The dominance and independence of England is maintained by the kings that follow.

It was not until 978 and the accession of Æthelred the Unready that the Danish threat resurfaces.

Two powerful Danish kings (Harold Bluetooth and later Sweyn, his son) both launch devastating invasions of England.

Anglo-Saxon forces are resoundingly defeated at Maldon in 991.

More Danish attacks follow, and their victories are frequent.

Æthelred's control over his nobles begins to falter, and he grows increasingly desperate.

His solution is to pay the Danes off: for almost twenty years he pays increasingly large sums to the Danish nobles in an attempt to keep them from English coasts.

Known as Danegelds, these payments slowly cripple the English economy and eventually become too expensive.

Northwest Europe (1012–1023 CE): Danish Domination, Irish Struggles, and Regional Transformations

England: Danish Conquest under Sweyn and Cnut

The period from 1012 to 1023 CE marked a dramatic turning point in England’s history. Following years of Danish raids led by Sweyn Forkbeard, tensions exploded after King Æthelred the Unready ordered the St. Brice's Day massacre. In 1013 CE, Sweyn launched a decisive invasion, rapidly overcoming English resistance. Æthelred fled to Normandy, allowing Sweyn to become the first Danish king of England. Sweyn's sudden death in 1014 briefly returned Æthelred to power but brought further turmoil.

In midsummer 1015, Sweyn’s son Cnut (later known as Canute the Great) launched a powerful new invasion fleet, landing at Sandwich, Kent, virtually unopposed. Cnut's forces quickly moved through Wessex, ravaging Dorset, Wiltshire, and Somerset. Eadric Streona, the influential but notoriously unreliable alderman, initially assembled an English fleet but promptly submitted to Cnut. Unopposed, the Scandinavian army advanced into Mercia and plundered northward into Northumbria, where its ruler, Uhtred the Bold, was murdered. Subsequently, Northumbria was granted to Cnut’s ally, Eric Haakonsson, who had left Norway around 1014 or 1015 specifically to support Cnut’s campaign.

When Cnut's army turned south again toward London in early 1016, Æthelred died, and his son Edmund Ironside ascended to the throne. Edmund briefly rallied English resistance, but after several fierce battles and an inconclusive siege of London, he agreed with Cnut to divide the kingdom. Edmund’s death just months later cleared the path for Cnut, who became undisputed king of England in 1017. Cnut divided England into four territories: Wessex he retained for himself; East Anglia he gave to the Danish chieftain Thorkell the Tall; Northumbria went to Eric Haakonsson; and Mercia was returned to Eadric Streona—who was swiftly executed later that year for treachery. Cnut’s reign thus established a Danish-dominated North Sea empire that significantly reshaped the region.

Ireland: Brian Boru’s Last Years and the Battle of Clontarf

In Ireland, tensions grew following High King Brian Boru’s divorce from his formidable wife, Queen Gormflaith, in the early 1010s. Gormflaith subsequently began orchestrating opposition to Brian, fueling rising discontent among the Leinstermen. Relations with Leinster had deteriorated so sharply that by 1012, open rebellion erupted, challenging Brian’s rule.

This opposition climaxed dramatically at the Battle of Clontarf on April 23, 1014. Brian’s forces confronted a broad coalition of rebellious Leinstermen and their Norse allies from Dublin and overseas. Although Brian’s army achieved victory, dramatically weakening Norse influence in Ireland, Brian himself was slain during the battle. His death removed a unifying figure, plunging Ireland back into political fragmentation.

Scotland: Stability Under Malcolm II

In Scotland, Malcolm II (r. 1005–1034) continued to solidify central authority. Throughout this period, Malcolm expanded his territories, notably securing control over Lothian. This consolidation provided critical stability and reinforced Alba’s internal coherence amidst the broader instability in Britain and Ireland.

Iceland: Consolidation of Christian Institutions

Following Iceland’s peaceful adoption of Christianity (1000 CE), the Icelandic Commonwealth further integrated ecclesiastical institutions and Christian laws into daily governance. The peaceful religious transition allowed Iceland to develop closer cultural ties with the European mainland without sacrificing the autonomy of its unique parliamentary system, the Althing.

Norse Settlements: Maritime Networks and Cultural Stability

Communities in Orkney, Shetland, and the Hebrides remained economically prosperous and culturally distinctive during this period, even as Norse power declined in Ireland after Clontarf. These island settlements continued as important maritime hubs, balancing cultural influences from Scandinavia, Ireland, and Anglo-Saxon England.

Literary and Cultural Developments

Around 1000 CE, the compilation of the Irish Dindseanchas cataloged legendary sites on the Hill of Tara, reinforcing the symbolic cultural unity Brian Boru sought. Meanwhile, around 950–960 CE, the compilation of the Cædmon manuscript in England reflected continuing vibrant intellectual and literary traditions, despite the turbulent political backdrop.

Legacy of the Era

By 1023 CE, Northwest Europe had undergone profound transformations. Cnut’s consolidation of power in England established a durable Danish presence, dramatically reshaping political alliances and territorial administration. Brian Boru’s reign and subsequent death at Clontarf marked a pivotal point for Ireland, sparking renewed political fragmentation exacerbated by personal rivalries and rebellions, notably those incited by Gormflaith. Meanwhile, Malcolm II’s stable governance in Scotland contrasted sharply with instability elsewhere. Iceland’s embrace of Christianity strengthened its European cultural alignment while preserving political independence.

These complex interwoven events set critical precedents, laying the groundwork for continued change in Northwest Europe throughout the eleventh century.

Ireland’s High King Brian Boru divorces Queen Gormflaith some time during the 1010s, and she begins to engineer opposition to the High King.

Relations between Brian and Leinster have become so strained that revolt breaks out among the Leinstermen around 1012.

Eric Haakonsson leaves Norway in 1014 or 1015 and joins Cnut, later known as Canute the Great, for his campaign in England.

The Scandinavian invasion fleet lands at Sandwich in midsummer 1015 where it meets little resistance.

Cnut's forces move into Wessex and plunder in Dorset, Wiltshire and Somerset.

Alderman Eadric Streona assembles an English force of forty ships and submits to Cnut.

The Scandinavian army moves over the Thames in early 1016 into Mercia, plundering as it goes.

Prince Edmund attempts to muster an army to resist the invasion but his efforts are not successful.

Cnut's forces continue unhindered into Northumbria where Uhtred the Bold, earl of Northumbria, is murdered.

The great north English earldom is given by Cnut to Eric after he wins control of the North.

The invading army turns south again towards London.

King Æthelred the Unready dies before they arrive, and Prince Edmund is chosen king.

The Scandinavian forces besiege London.

After several battles, Cnut and Edmund reach an agreement to divide the kingdom, but Edmund dies a few months later.

Cnut, as the undisputed king of all England in 1017, divides the kingdom into four parts.

Wessex he keeps for himself, East Anglia he gives to Thorkell, Northumbria to Eric and Mercia to Eadric.

Cnut has Eadric executed as a traitor later in the same year.

Cnut returns in August 1015 to find a complex and volatile situation unfolding in England.

Æthelred's son, Edmund Ironside, has revolted against his father and established himself in the Danelaw, which is angry at Cnut and Æthelred for the ravaging of Lindsey and is prepared to support Edmund in any uprising against both of them.

Cnut conquers most of England over the next months.

Edmund had rejoined Æthelred to defend London when Æthelred dies on April 23, 1016.

He is buried in old St. Paul's Cathedral, London.

According to the Anglo-Saxon Chronicle: "Then collected he [Edmund] his force the third time, and went to London, all by north of the Thames, and so out through Clayhanger, and relieved the citizens, driving the enemy to their ships.

It was within two nights after that the king went over at Brentford; where he fought with the enemy, and put them to flight: but there many of the English were drowned, from their own carelessness; who went before the main army with a design to plunder.”

The subsequent war between Edmund and Cnut ends in a decisive victory for Cnut at the Battle of Assandun on October 18, 1016, after Edmund is betrayed on the field by the Ealdorman of the Mercians.

Edmund's reputation as a warrior is such that Cnut nevertheless agreed to divide England, Edmund taking Wessex and Cnut the whole of the country beyond the Thames.

However, Edmund dies on November and Cnut becomes king of the whole country.