West Frankia suffered most from Viking depredations, its great rivers giving Viking fleets access deep into the heart of the kingdom. In the 860s, Charles developed an effective strategy of building fortified bridges and forts to deter the Vikings; he also hired Viking leaders. In the mid-860s, most Viking bands moved to England, aware that there were easier pickings to be had there.

Within ten years of their arrival in England, only Wessex remained fully independent of the Vikings, and parts of the Great Army began to settle in Northumbria and Mercia. They failed to overcome Alfred of Wessex, but agreed to a frontier which nevertheless left half England to the Vikings. Part of the army then settled in East Anglia. The remainder crossed to Frankia.

The failure of the Great Army’s attack on Wessex in 878 coincided with renewed confusion in the Frankish kingdoms. After reinforcements arrived in 878, the Great Army crossed to Frankia, devastating new areas in Flanders, raiding deep into the Rhine valley in 882, and in 885 moving on to the Seine valley. Although they failed to capture Paris in the winter of 885 -86, the Vikings did go on to plunder Champagne and Burgundy, which had hitherto escaped.

When the Great Army left England in 879, the Danes in East Anglia, east Mercia, and at York remained a potential threat to Wessex. This added urgency to Alfred’s reform of West Saxon defences. When the Great Army returned in 892 it met a vigorous response. In 896, the Great Army recognized that Wessex could not be conquered, nor even plundered, and it finally broke up.

From 865, Viking activity in the west was concentrated in a ‘great army’. From 882-92 the Viking Great Army (magnus exercitus) – ‘everyone called it that on account of its numbers’ (Miracles of St Bertin) – operated on the continent. When it crossed to England it was also called the Great Army. It began its activities in England, and whilst there the raids in Ireland and Frankia abated. It was made up of fleets led by several kings. Squadrons departed and new ones arrived, resulting in continuous activity over three decades.

The Great Army introduced a new tactic in 865: each autumn it seized a significant inland centre from which to plunder a fresh district. Most of these sites already possessed Roman walls or later earthworks; if not, defences were improvised, as at Reading where a ditch and bank fortified a promontory between two rivers. The Army relied on horses for mobility – it was ‘horsed’ as soon as it arrived in 865 – and although its bases were mostly on navigable rivers, little use was made of the fleet. Its leaders were ambitious. Puppet regimes were established in Northumbria, East Anglia, and Mercia, then parts of the army shared out the land. They were replaced by new contingents in 871 – a ‘great summer fleet’ (Anglo-Saxon Chronicle) – at Reading, and in 876, and 879.

The Anglo-Saxon Chronicle (in fact a West Saxon source) gives the impression that for a decade the Anglo-Saxon kingdoms put up ineffectual resistance before paying tribute, and only Wessex led by Alfred resisted effectively. During the invasion of 870-71, the West Saxons fought nine battles against the Great Army, winning two, and many skirmishes against raiding parties. Despite killing one Scandinavian king and nine earls, in the end king Alfred (871-99) made peace and doubtless paid tribute too, as his neighbours had done.

From 875 to 878, Wessex, the only kingdom not in the Great Army’s power, endured a sustained assault. How King Alfred survived is not clear. Like Charles the Bald, he used a containing strategy against the Great Army in Wareham (875-76) and Exeter (876-77), forcing it to move on. When the Vikings then made a surprise attack in January 878 and Alfred was temporarily deserted, he had the tenacity to keep up the struggle. He defeated the Great Army a Edington (June 878), then blockaded it in Chippenham until king Guthrum made peace. Alfred was fortunate. By 878 the Great Army was weakened by departures; the fleet which arrived in 876 lost many ships in a storm; and in 878 another fleet only arrived after his victory over Guthrum.

It was renewed discord in Frankia that beckoned the Great Army: after the deaths of the experienced rulers Louis the German and Charles the Bald (876, 877), six kings shared the kingdom in a decade – Louis the Stammerer (877-79), Louis III (879-82), and Carloman (879-84) in West Frankia, Carloman (876-80), Louis the Younger (876-82), and Charles the Fat (876-87) in East Frankia. From 879 the Great Army systematically scoured Middle Frankia, which had experienced few raids in the 850s and 860s and whose towns and monasteries lacked defences. In 880, armies from West and East Frankia failed to trap the Great Army on the ScheIdt, but Louis III defeated a raiding force at Saucourt in August 881. The construction of a fort at Etrun to block the ScheIdt persuaded the army to move to the Meuse. Charles the Fat besieged their camp at Ascloha (Asselt?) in July 882, but after only two weeks he bribed the army to return to West Frankia, to Conde. The West Frankish ruler Carloman defeated a party near Rheims, recovering its plunder, but could not prevent further raids and finally paid the Army to depart in 884. Now the Great Army divided: part crossed to England to besiege Rochester unsuccessfully; part was defeated at Louvain in 885 and besieged by East and West Frankish forces, finally escaping by night.

In Flanders and Lotharingia, little impeded Viking pillaging before 885. Local forces (led by counts, bishops, and abbots) were generally beaten, and while larger royal armies could defeat raiding parties, victories like Louis Ill’s at Saucourt had little permanent effect. The Viking raiders’ mobility made interception difficult as they were willing to use terrain like forests to escape. By 885, however, the area had been thoroughly ravaged and Carloman’s death without an heir in December 884 made the Seine valley, after a twenty-year break from serious raids, an inviting target. In July 885, the Great Army reunited in the Seine. The bridge at Pont de l’Arche failed to stop them, but that at Paris was strongly held, even though the defences were incomplete when the army arrived. The town stood on an island connected to the banks by bridges defended by two forts. The Vikings failed to storm the northern fort (26 November), so they fortified winter quarters from which to maintain the siege and pillage the surrounding region for the next year. The siege was the subject of a long poem written by Abbo, an eye-witness; although prone to exaggeration – he estimated the Great Army as 40,000 men and 700 ships, resisted by a mere 200 Franks – he gave valuable details, such as descriptions of the siege weapons built for the Vikings by renegade Franks, and the use of a fire ship against the bridge. Paris resisted all the Viking attacks, but nor could the Franks dislodge the Vikings from their camp.

Charles the Fat ended the deadlock by making another humiliating treaty: the Great Army was paid tribute and allowed up-river to winter in Burgundy, the very thing the bridge at Paris had been intended to prevent. For three years it pillaged in the upper Seine basin, ravaging towns and monasteries hitherto untouched, such as Verdun, Toul, and Troyes. Charles’ failure led to his deposition. In 889 the new West Frankish king, Odo, checked the Vikings near Paris and bribed them to move on. Defeated by the Bretons at St Lo (890), they made again for Flanders. There the respite from Viking attack had been put to good use: in the 880s town walls had been restored, monasteries fortified, and forts built for the population. In the winter of 890-91, the Great Army had to besiege Noyon, and a force which assaulted an earth and timber fort neat St Orner, one of the ‘forts that had been recently built’ (Miracles of St Bertin), was repulsed. Haesten’s army from Amiens likewise found Arras held against it. In spring, the Great Army moved to Louvain and dug in between the river Dyle and a marsh, and the East Frankish king Arnulf stormed the camp (the battle of the Dyle). East Frankish writers duly claimed a decisive victory, yet the Great Army soon reoccupied the camp. The campaigns in Frankia had followed the familiar pattern of fortifying winter quarters and raiding by horse all year round. The ships seem to have followed from camp to camp; 200 are said to have left Ascloha on the Meuse in 882. Sometimes they had a separate base, but were readily available when required. Godemd, one of two kings leading the Great Army, left in 882 when Charles the Fat granted him land in Frisia. When Haesten’s fleet arrived from the Loire, forced out by Louis III in 882, it kept its identity and in 890-92 had a separate base. What finally made the Great Army quit Frankia was not resistance but a famine.

In autumn 892, the Vikings crossed to Kent (map 6) the Great Army ‘in one journey, horse and all’ (Anglo-Saxon Chronicle), in an estimated 200 or 250 ships, Haesten’s fleet in 80 – and established winter bases. For four years these Vikings, with the Danes already settled in England, raided into Wessex and the rump of Mercia (now close allies). However, Alfred had used the 880s to reorganize the defences of Wessex. He had divided the available warriors so that while some were on garrison duty, of the remainder ‘always half its men were at home, half on service’ (Anglo-Saxon Chronicle). He had created a network of fortresses, burhs in Old English. These were refuges, but their garrisons formed rapid reaction forces against Viking raids. The result was that the Vikings were hotly pursued wherever they went.

During the winter of 892-93, Alfred kept a close watch on the two Viking bases. When the Great Army moved into Wessex, in the spring of 893, his son Edward defeated it at Farnham and beSieged the Vikings on an island in the Thames. Then, while Alfred led an army to Exeter, besieged by a fleet from the Danish settlements, Edward’s army stormed Haesten’s new base at Benfleet in Essex. A third raid by the Great Army in 893, up the Thames and Severn, was pursued by troops from West Saxon and Mercian burhs and Welsh princes. They besieged the Vikings at Buttington, where allegedly they were forced to eat their horses, and defeated them, although this did not prevent the Vikings from returning to Essex. Alfred’s division of his forces had proved equal to the challenge in 893, but nonetheless the Great Army regrouped and mounted another raid in the autumn, after the Vikings ‘had placed their women and ships and treasure in safety’ (Anglo-Saxon Chronicle). The pursuing English could not prevent them from occupying the Roman fortress of Chester, but by clearing the grain and cattle from the surrounding area they forced the army to winter in Wales instead.

After the hectic campaigning of 893, the next two years appear calmer in the Anglo-Saxon Chronicle. In autumn 894, the Great Army rowed up the Thames and Lea to Hertford and established new winter quarters, remaining there until August 895 when Alfred positioned an army to deny the harvest to the Vikings. In a move reminiscent of Charles the Bald in 862, he built a fortified bridge to block the Lea. This forced the Great Army to abandon its ships. It moved to Bridgnorth on the Severn for the winter of 895-96, a safe distance from Wessex.

The following summer the Great Army broke up, some settling in eastern England, the remainder crossing to West Frankia. That summer Danish settlers raided the West Saxon coast and tested the ships Alfred had designed, ‘built neither on the Frisian nor the Danish pattern, but as it seemed to him they could be most useful’ (Anglo-Saxon Chronicle). They were not an unqualified success. In contrast to the 870s, Wessex itself had hardly been touched by the Great Army raids of 892-96. Part of the explanation must lie in the network of defended forts Alfred had established. Tight marking by forces from Wessex and Mercia, matching the Vikings’ mobility, had been able to prevent the Great Army from operating profitably. After thirty years of pillaging it finally dispersed. Those who had capital joined the settlers in eastern England; those without returned to the Seine where they established the nucleus of the future Normandy.