From random site I googled
Quote:The earliest documented raids by the Vikings began in 793 at Lindisfarne, England. Historians distinguish three phases to the 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.