The first recorded Viking raid upon the British Isles occurred in 793 C.E., during the reign of King Beorhtric of Wessex. Simeon of Durham recorded the grim events:
"And they came to the church of Lindisfarne, laid everything waste with grievous plundering, trampled the holy places with polluted feet, dug up the altars and seized all the treasures of the holy church. They killed some of the brothers; some they took away with them in fetters; many they drove out, naked and loaded with insults; and some they drowned in the sea."
The Anglo-Saxon Chronicle for the year 793 records:
"In this year terrible portents appeared over Northumbria and sadly affrightened the inhabitants: there were exceptional flashes of lightning, and firey dragons were seen flying in the air. A great famine followed soon upon these signs, and a little after that in the same year on the ides of June the harrying of the heathen miserably destoyed God's church in Lindisfarne by rapine and slaughter".
Within the next five years, Viking raiders would strike at Lindisfarne and Jarrow in Northumbria, at Wales, Ireland, the Isle of Man, the Isle of Iona in Scotland, and even the islands off Aquitaine in France. Over the next 250 years, the Vikings became justly feared throughout Europe as the Hammer of the North.
From the beginning of the sixth to the end of the eighth century, the European mainland was more or less free of external invasion. Early in the ninth century, however, this peace came to an end.
Historians distinguish three phases to the Viking raids. The first phase of attacks was from 790-840. The Vikings used shallow draught longships which were ideally suited for surprise raids on coastal locations that struck terror into their victims. The fleets were small, making a "hit-and-run" tactic of the attacks that could enable the raiders to row away as swiftly as they had come. The attacks were usually seasonal and isolated in small bands. These attacks began along the coastal cities in England and France and continued down along the river communities. The Carolingian Empire was deeply affected by the raids at this time. Frisia and Aquitaine in modern day France were two of the first provinces attacked by the Vikings, Aquitane being attacked by Norwegian raiders returning from Ireland. The most notable attack was on the monastery at Noirmountier. This island monastery was attacked every summer. The monks tried many defenses, but they eventually left the island for safer lands. The trading centers in Frisia, particularly Dorestad, were a favorite targets of the Vikings in 834-839.
During the second phase of Scandinavian activity from 841-875, the raids increased in number, size, intensity and speed. By 851, the fleet ships had increased from 3 ships to 350 ships per raiding party. The Vikings arrived, unexpectedly, by plundering, burning, killing or enslaving the inhabitants and then leaving the conquered lands. This war tactic accounted for the Vikings' great success in this period. They met no organized resistance, but the Vikings were defeated here and there from particular clan groups. New hordes came to fill the gap, or they turned their attention elsewhere. In 843, the Viking warriors wintered on foreign soil for the first time. They settled at Aquitaine, and this place was never completely free of Vikings. Gradually, the Viking attacks moved from English and French soil to the Mediterranean Sea. In 844, a fleet attacked Nantes, Toulouse, Gijon, Lisbon and Seville. This fleet was defeated, and returned to Aquitaine. A second fleet reached even further, raiding North Africa, France and Spain, and then continued on to Italy where it was defeated. The Vikings formed the "Great Army" that consisted of thousands of individuals. This was an important military achievement for the Vikings during these years. The leaders continued to change, and different bands raided different areas. The war-bands increased in size, and each war-band fought for itself. Occasionally, the armies even fought against each other. As they expanded their conquests, the invaders also began to leave their ships and travel on foot or on horseback.
By remaining on foreign soil, the Vikings increased the political threat to the local rulers. Some Viking parties joined forces with enemies of the kings or rulers. Many Anglo-Saxon and Frankish rulers bought off the Vikings in an attempt to remove them from their lands. In 862, Charles the Bald tried to fortify the bridges to stop the passage of the Viking fleets, but it remains unclear if these bridges were successful.
In the third phase between the years of 876-911, the Vikings, along with their Great Army, continued to plunder on both sides of the Channel and began to colonize England and France. They also permanently settled in lands they had raided such as Ireland, Iceland, and areas in Russia around Novgorod and Kiev. The military response from the conquered peoples varied from one ruler to another. In some places, the Vikings met great opposition from the people. After suffering devastating blows from the Vikings, the English army reorganized: half its men were home and half out on service. A new type of craft was constructed which could oppose the Viking longships in shallow coastal waters. Therefore, when the Vikings returned from the continent in 892, they could no longer roam the country at will due to opposition by the local army's counterattack. Charles the Simple, king of the West Franks, ended the Viking raids in 911 by giving Normandy to the Vikings. In return, Rollo, a Viking leader, pledged his allegiance to Charles, was then baptized, and defended the lands against other Viking parties.
It's worth noting that raids similar to those conducted by the Vikings occurred in other parts of Europe during the Viking era. What made the Viking raids so notable was their success (due in large part to the superiority of Viking ships) and their extent (well outside the borders of the Norse lands).
The raids were only one aspect of the Norse expansion. It's not clear what triggered this outward movement at the end of the 8th century. Perhaps it was due to population pressure, since portions of Scandinavia were overpopulated by the standards of the time.
Dudo of St. Quentin's Gesta Normannorum is probably the earliest attempt to explain the Viking expansion. Written around 1020AD, the part which describes the factors that Dudo believed led to the Viking settlements and raids is in chapter 2:
http://orb.rhodes.edu/ORB_done/Dudo/chapter02.html
There is a *lot* of scholarship out there discussing the whys and wherefores of the Viking expansion. If your school library doesn't have these books, check the public library. You can also usually go to a college or university library to use a book on the premises, though you won't be able to check it out -- college libraries also have xerox machines, so you can photocopy key information.
Some excellent discussions of the Viking expansion:
Fitzhugh, William W. and Elisabeth I. Ward, eds. Vikings: The North Atlantic Saga. Washington DC: Smithsonian Institution Press. 2000.
http://www.amazon.com/exec/obidos/A...hevikinganswerl
[This is the companion book to the travelling "Vikings!" exhibit from the Smithsonian.]
Magnusson, Magnus. Viking Expansion Westwards. London: The Bodley Head. 1973. Out-of-print, to have Amazon.com do a book search for it go to:
http://www.amazon.com/exec/obidos/A...hevikinganswerl
Sawyer, P. H. Kings and Vikings: Scandinavia and Europe, A.D. 700-1100. New York: Methuen. 1982. Reprint, 1993. To order from Amazon.com:
http://www.amazon.com/exec/obidos/A...hevikinganswerl
[Discusses the Viking expansion westwards, including raids and piracy affecting Western Europe, with an assessment of the consequences, good and bad, that this activity had on both the Vikings and their victims.]
Jones, Gwyn. A History of the Vikings. Oxford: Oxford University Press. 1968. To order from Amazon.com:
http://www.amazon.com/exec/obidos/A...hevikinganswerl
Foote, Peter and David M. Wilson. The Viking Achievement. London: Sidgewick and Jackson. 1970. Out-of-print, to have Amazon.com do a book search for it go to:
http://www.amazon.com/exec/obidos/A...hevikinganswerl
Pulsiano, Phillip et al., eds. Medieval Scandinavia: An Encyclopedia. Garland Reference Library of the Humanities 934. New York & London: Garland. 1993. Out-of-print, to have Amazon.com do a book search for it go to:
http://www.amazon.com/exec/obidos/A...hevikinganswerl
Lund, N. "Allies of God or Man? The Viking Expansion in a European Perspective." Viator 20 (1989): 45-59.
Also see:
http://viking.hgo.se/Files/VikHeri/.../expansion.html
http://www.valhs.org/history/articl.../text/raids.htm