The Battle of Ashdown, fought on 8th January 871 in southern England, saw the Kingdom of Wessex defeat the Great Heathen (Viking) Army. This was the first major defeat of the Viking army, which had arrived in England in 865 and had conquered several Anglo-Saxon kingdoms. While the conflict between the Wessex and the Vikings continued for several months into 871 and beyond, Ashdown demonstrated that the Viking army could be defeated, giving the English hope for future battles.

Wessex at the time was ruled by King Aethelred (reign 865 to 871), but his younger brother – and soon-to-be successor – Alfred the Great (reign 871 to 899), played a decisive role in the victory at Ashdown, proving himself a capable military commander and ensuring the West Saxon nobles backed his succession later in 871. Alfred spent most of his reign locked in a struggle with the Vikings, but secured his kingdom through victories at the Battles of Edington in 878 and the Battle of Buttington in 893.

Our knowledge of Ashdown relies heavily on The Life of King Alfred, a contemporary biography of the king written by his advisor and tutor, Bishop Asser. It presents a pro-Alfredian version of Ashdown, yet it nevertheless provides the most detailed contemporary account of a battle in 9th-century Britain, offering invaluable insights into Anglo-Saxon warfare.

England in the 8th century was divided into several kingdoms: East Anglia, Mercia (Midlands), Northumbria (northern England), and Wessex (southwest England). The rulers of these kingdoms competed for wealth, land and prestige, often fighting wars and occasionally allying with one another. It was this world of Anglo-Saxon game of Thrones that the Vikings entered into toward the end of the century. Initially, they began as small groups of Scandinavian pirates, sailing south in the hope of wealth and plunder. They first attacked Wessex in 789 and then Northumbria in 793, where they notoriously raided the prestigious monastery of Lindisfarne. With their great stores of wealth and limited defences, coastal monasteries and trading ports quickly became favoured targets for Viking raids in Britain.

Around the same time, the Kingdom of Wessex was growing in power, and upon conquering Mercia-ruled Kent in 829, it became the most dominant of the Anglo-Saxon kingdoms. While its early 9th-century kings, Egbert (reign 802 to 839) and Aethelwulf (reign 839 to 858), were effective military leaders and enjoyed several victories against the Vikings, these raids were becoming increasingly difficult to halt. Even their wealthiest towns soon became valid targets, and around the middle of the 9th century, Canterbury, London, Southampton and Winchester were looted by the Vikings.

Using hit-and-run tactics, the Vikings could sail toward and plunder a coastal target, then return to their ships, before the English – who did not have standing armies – could summon numbers large enough to fight the raiders. They would, however, not long remain content with mere plunder; they soon wanted to conquer and settle in England, too.

In 865, an alliance of sea-kings and raiders – known by contemporaries as "the Great Army" – gathered in the North Sea. According to the Norse sagas, its leaders, Ivar, Halfdan, and Ubbe, were brothers seeking revenge for their father, who had been killed by an English king. They were remembered by the English simply as "valiant but cruel men." East Anglia – the smallest of the English kingdoms – was their first target. However, King Edmund of East Anglia (reign 855 to 869) offered tribute to the Norsemen in exchange for a peace treaty. The invaders were provided with horses, silver, food supplies and were allowed to winter in East Anglia.

The following year, their new equestrian companions were put to use as they rode into Northumbria, conquering York, its wealthy capital. Two Northumbria kings led attacks to reclaim the town, but along with several northern ealdormen (earls), they were defeated and died at York. One shrewd Northumbrian ealdorman, Egbert, however, had stayed clear of battle and aligned himself with the invaders. In exchange, he was appointed to rule over the northern regions of Northumbria (historically known as Bernicia) on their behalf. Meanwhile, the Viking took direct control of York and its surrounding lands.

In 868, they turned south, capturing Nottingham in northwest Mercia and fortifying themselves within. The Mercian ruler, King Burgred (reign 852 to 874), had long been an ally of the West Saxons and thus requested help from King Aethelred of Wessex. Together, they placed Nottingham under siege, but for the Anglo-Saxon commander, fortress warfare was an uncommon experience. They were unable to attack Nottingham's walls directly, nor could they maintain their armies long enough to starve Ivar into submission. Therefore, the Vikings accepted tribute payment from Burgred, delivered in gold and silver, in exchange for peace. Each payment, no doubt, emptied the coffers of its payees and attracted new recruits from Scandinavia, who heard tales of the prosperity enjoyed by their kinsmen and friends in England.

The following year, the Vikings returned to East Anglia, capturing Thetford, but no amount of tribute would save Edmund this time. His army was defeated, before he was tracked down and brought before Ivar. Edmund was offered to be the Vikings' vassal, like Egbert of Northumbria, and upon his refusal, he was tied to a tree before being stabbed with spears and beheaded. Burgred of Mercia might now have understood the hollowness and temporary nature of a peace treaty with the Vikings.

Ivar was seemingly content with his work so far. Establishing dominance of England's east coast and the great trading centre of York, he next took his followers north to fight the Strathclyde Britons. This left Halfdan as the senior leader of the army in East Anglia. Known by the West Saxons as "the tyrant," Halfdan was a Danish ruler who left behind his kingdom to carve out a new one amongst the British Isles (Aethelweard, 41). With Northumbria crushed, Mercia taxed and King Edmund martyred, Halfdan decided Wessex would be next.

In December 870, he marched southwest from East Anglia, taking Reading on Wessex's northern border. The town was ideally situated as an invasion headquarters. Sitting on the Thames, it allowed new soldiers and supplies to flow in and provided an easy escape route if things went wrong. With the town surrounded on two sides by the Thames and the River Kennet, Halfdan built ramparts around the rest of Reading, making it an impregnable fortress.

Upon arrival, a raiding party was sent out to the Berkshire countryside in search of plunder, slaves and supplies. All the while, a local nobleman and veteran of Viking warfare, Ealdorman Aethelwulf of Berkshire, was rallying his troops. His army fell upon the raiders at Englefield (5 miles or 8 km west of Reading), cutting them down from their horses and slaying their leader. A minor battle it might have been, but the first victory had gone to Wessex.

To the south, more support awaited as King Aethelred was raising a large army to help Ealdorman Aethelwulf. As Aethelred and Aethelwulf came to the fortress, they slaughtered several unprepared Viking soldiers found outside its walls. Yet, taking the fortress was altogether a different task. The two leaders, Aethelred and Halfdan, had already met previously, during the siege of Nottingham in 868, when the Vikings were content to play the waiting game, before being paid to leave. Halfdan, however, would not repeat this strategy. He surprised his opponent and immediately confronted the West Saxons. As Asser explains:

Like wolves, they burst out of all the gates and joined battle with all their might. Both sides fought there for a long time, and fought fiercely, but alas, the Christians eventually turned their backs, and the Vikings won the victory and were masters of the battlefield.

(Keynes and Lapidge, 78)

Amongst the dead outside Reading was Wessex's most celebrated war chief, Ealdorman Aethelwulf, a great blow to the king's campaign. Aethelred's army retreated westward, perhaps linking up with fresh soldiers arriving from Wiltshire and Somerset. Halfdan now felt he had the perfect opportunity to finish off Wessex for good. His experience in East Anglia and Northumbria taught him that the capture of a key town, followed by military victory and the king's death, led to the kingdom's surrender. So, he followed Aethelred west, hoping to inflict a second and fatal defeat upon him.

On 8th January 871, on a cold winter morning, the armies of Aethelred and Halfdan caught sight of one another amongst the northern Berkshire hills. However, where they stood is uncertain. "Ashdown" is often a term used to describe the whole of the northern Berkshire Downs, rather than a specific place. Yet most scholars suggest the battle likely took place among the hills west of the village of Moulsford, on the Oxfordshire-Berkshire border.

We also know nothing of either army's size. While the West Saxons would have lost many outside the gates of Reading, fresh recruits from the west after the battle likely brought them back to a size similar to the Viking army, which had left behind a garrison to guard Reading.

The army of Wessex was formed from a core of professional warriors serving in the household of the king and ealdormen, along with those of lesser status who owed military service to and were led by local landowning warriors (thegns). The soldiers on both sides were mostly armed with shields and spears, with axes more prevalent among the Vikings, and the elite West Saxon warriors bearing swords—an expensive, rare weapon often passed down through generations.

The Vikings had arrived first at the battlefield, taking the higher ground at Ashdown. As the two sides looked across from one another, Halfdan gave his first order. His army was to be divided into two, half led by him and his ally, King Bagsec, and the other half by several earls. As they spread out across the upper grounds of the Berkshire hills, Halfdan gave a second order, and his army duly formed a "shield wall" – a long front line of soldiers locking their shields together to guard against an incoming attack. Seeing this, Aethelred followed suit, splitting his army into two shield walls, with himself commanding one to oppose Halfdan and Bagsec, and his younger brother, Prince Alfred, commanding the other to face the Viking earls.

However, before any fighting could begin, Aethelred returned to his tent for a pre-battle mass, praying for divine help. Some form of prayers or mass was not unusual before battle. The English were, after all, convinced that the Vikings had been sent to punish them for their sins. Holding himself responsible for the souls of his subjects, Aethelred was fighting both a physical and a spiritual war.

Halfdan, however, had no interest in respecting Christian pre-battle rituals. He seized this moment of vulnerability to strike first. Abandoning the high ground, he and the Viking earls marched on Alfred, hoping to crush his army with their superior numbers. When Aethelred was informed, he declared he "would not leave that place alive before the priest had finished mass and that he would not forsake divine service for that of men" (Keynes and Lapidge 79). Alfred would now have to take on both Halfdan and the Viking earls alone. "With loud shouting from all," and Alfred "acting courageously, like a wild boar," as Asser tells us, he strode forth to meet the Vikings (Keynes and Lapidge, 79).

The clash between the two armies sounded with a mighty "thud" as the walls of wooden shields smashed together. A shield wall versus shield wall clash often sank into a slow-moving shoving match. With little innovative strategy or creativity, in this battle of strength, power and determination, each side would seek to push the other back and break through their wall. Meanwhile, frontline warriors poked, jabbed, prodded and stabbed their spears at gaps in the wall, hoping to deliver lethal blows to their opposite number. Such a slow-moving contest suited Alfred; being vastly outnumbered, he could only hope to hold off the Vikings long enough for Aethelred to come to his rescue.

Eventually, Aethelred arrived, leading his troops in an ambush of the Vikings. Some scholars have even suggested that his delayed entry was a tactical move, rather than a religious one. Perhaps striking the Vikings' flank or rear, he relieved pressure on his brother and gained the upper hand over the invaders.

The fight continued for several more hours, yet as the waning Viking army was attacked on two fronts, their shield wall crumbled and collapsed. The West Saxons surged through the breaches, hewing down their foes in great numbers. A 10th-century chronicler, Ealdorman Aethelweard, remarked, "neither before nor after has such a slaughter been heard of." Asser recorded:

Bagseg was killed, and Earl Sidroc the Old, Earl Sidroc the Younger, Earl Osbem, Earl Fraena, and Earl Harold . ... the entire Viking army was put to flight, right on till nightfall and into the following day, until such time as they reached the stronghold from which they had come. The Christians followed them till nightfall, cutting them down on all sides.

(Keynes and Lapidge, 80)

Halfdan's plans of a quick victory had been shattered. Just over a week into the war, a large chunk of his army and several of his leading allies had died. Wessex was not, after all, so weak as Northumbria or East Anglia. However, given his experience at Reading just a few days before, Aethelred would not assault the town again. It appeared that the conflict would be a stalemate. The Vikings were unable to conquer Wessex, and the West Saxons were unable to expel the Vikings.

However, with the death of several allied Viking leaders – and potential rivals for command – Halfdan now commanded greater authority over the army. Perhaps to secure his complete leadership over this admittedly smaller force and replenish supplies, only two weeks after Ashdown, he led another raid into Wessex. His target was Basing, 15 miles (24 km) south of Reading. Outside the town, he was once again met by King Aethelred's army, but this time he defeated the West Saxons and took control of Basing. The conflict now reached an informal ceasefire, with two months of peace. The campaign was reignited in March, when the Vikings raided deep into Hampshire, where they defeated Aethelred at the Battle of Merton. Following the battle, a second Viking fleet (known as "the great summer army") sailed from the continent to Reading to assist Halfdan.

Worse, during the battle at Merton, Aethelred had been fatally wounded. He only lived a few more weeks before dying in April 871. In accordance with arrangements made earlier that year, Alfred succeeded to the throne, ahead of his brother's infant sons, Aethelwold and Aethelhelm, who were not yet old enough to rule. Although he frequently suffered from a mysterious and severe illness, Alfred was 22 years old and had already proven his bravery during the battles at Reading, Ashdown, Basing and Merton.

However, his reign began with a series of military disasters. While he was overseeing his brother's funeral and burial at Wimborne Minster, the army of Berkshire was defeated by the great summer army at Reading. Nearer to home, in May 871, Alfred himself was defeated in battle by Halfdan near Wilton, Wiltshire. This had dashed any hope Alfred had of a military solution. The only way the Vikings would now leave was by paying them. The payment (or "danegeld") would no doubt empty his coffers and make him unpopular with his new and now heavily taxed subjects.

While Halfdan had enjoyed a string of victories, he had not won a decisive enough victory to discredit support for Alfred. Nor had he intended to lead such a long campaign; thus, as his army grew smaller with each battle and supplies became ever more meagre, he accepted being paid to leave Wessex, retreating to London in the autumn of 871. As the year ended, Alfred could take solace in the fact that he kept his crown and kingdom, a fate better than Edmund of East Anglia or the Northumbrian kings.

871 had been a year of great significance for the Viking invasion. Significant death tolls were suffered by both sides, including several of the leaders involved. Wessex had, however, shown it was capable of defeating the Great Heathen Army in battle, something that no other kingdom had achieved so far.

The war between the English and the Vikings would continue. Halfdan attacked Mercia in 874, forcing King Burgred into exile on the continent. He would subsequently shift his focus to Northumbria and Ireland, leaving the leader of the great summer army, Guthrum, to take up the fight against Alfred in the south. When Guthrum invaded Wessex in 878, he chased Alfred into the Somerset marshes, almost conquering the entire kingdom. Only several months later, when Alfred regathered the West Saxons, was he able to raise a large enough army to defeat Guthrum at the Battle of Edington.

Guthrum and Alfred soon agreed to a more lasting peace treaty, with the Viking leader converting to Christianity and southern England being divided between them, as the declining Mercian kingdom was carved into an English west and a Norse east. As Christian brothers, they endeavoured to uphold an imperfect peace between both sides until Guthrum died in 890. However, the next generation would continue the fight for supremacy, with Alfred's heirs – Edward the Elder (his son) and Aethelstan (his grandson) – extending West Saxon control over the rest of England.