Warmatrix

The vikings were seafarers from Scandinavia, who raided, traded and migrated across much of Europe's coasts and rivers during three centuries. They were not a single people, but shared a common Old Norse culture and language. As warriors, they were very successful.
In history the vikings have met a lot of bad press, because they often attacked monasteries and other church strongholds, were the only people were based who wrote historical accounts: the monks. The vikings attacked churches and monasteries not because of religious strife, but because these were full of loot. Most of them simply were in it for the money. Their first planned raid, in 793 CE, was aimed at the monastery at Lindisfarne, England. Early attacks were done by small bands of adventurers with a handful of ships. When these proved successful, more vikings acquired a taste for raiding. The number of attacks increased and around 820 CE the Norsemen and Danes started to establish seasonal and permanent bases, like the Isle of Man.
Viking raiders often attacked in late autumn, when the harvest was in and defending armies started to disperse at the end of their campaigning season. They crossed the seas in their longships, which allowed them to land wherever they chose, denying the defenders any early warning. Often they rowed up rivers to penetrate inland, made possible by the shallow draft of their ships. Once disembarked, they quickly seized all horses they could find, transforming themselves into mounted infantry, because they still fought on foot. The speed of their attacks made for perfect raiding. Instead of fighting their way in, the Vikings were usually gone before any defenders arrived. This does not mean that Viking attacks were lightning strikes; some of them took weeks or even months. But they still moved faster than their land-based enemies, who mobilized agonizingly slow. In cases when the Vikings lingered around longer, they often camped in fortifications like fortresses, churches or simply islands in rivers. These were used both strategically as winter quarters and tactically as bases to strike from and retreat to if they met strong resistance. Vikings did not sit inside these waiting to be besieged, but made sorties and other counterattacks as soon as an opportunity presented itself. The Franks and Anglo-Saxons sometimes tried to buy these semi-permanent raiders off, but that only provoked follow-up attacks to gain more extortion money.
Warfare appealed to the vikings, who loathed stealthy thieves but praised warriors who gained plunder in open fights. This war culture made sure that every man was at least somewhat of a fighter, though their prowess varied. Especially young men, the "drengr", were expected to raid, then to come home with wealth and honor. When this proved successful, the expeditions grew larger and in a few cases the vikings switched from raiding to conquest. Raiding bands were usually quite small, but invading armies seem to have numbered 150 - 250 ships and a few thousand men. The Danish armies of the 11th century CE were made up of professional warriors, paid by the king and quartered in military camps. The best of these were the "hirðmenn", who formed the bodyguards of jarls and kings.
In battle, the experienced vikings often outmatched their enemies in both fighting prowess and cunning, though certainly not always. The main effort of most viking battles was to break through an enemy 'shield wall' or hold it against the other side. But they also employed ambushes, sorties from fortresses and other tricks. Some battles started by hurling a single spear towards the enemy, to dedicate victory to Odin, the first among the Norse gods. Another viking phenomenon were the berserkers, who whipped themselves into a battle frenzy and attacked without regard for their own safety. Early vikings were small bands that were lightly armored, in cloth or leather. Their arms were mostly spears and round wooden shields. The wealthiest warriors wore mail armor and wielded swords, though these were quite rare. Later in the Viking age mail, bows and that typical viking weapon, the battle axe, became more common.
Not all vikings were raiders and not all raiders raided all the time. They Vikings resorted to plundering if they could, but also traded a lot, especially in the east, where the Swedes traveled up and down the rivers as far as the Black Sea. Several times they attacked Constantinople, without success. But the Byzantine emperor was so impressed with their prowess that he hired a number of them to form a royal bodyguard, the Varangian Guard. In the west raiders were followed by conquerors, especially from Denmark. Their 'Great Army' was halted by Alfred the Great at the Battle of Edington in 878 CE, after which the Danelaw pact was established, making north and middle England Danish territory. Much later, in 1018 CE, king Knut the Great briefly gained the English throne, setting up an Anglo-Danish kingdom. In the wake of the armies came migrants who settled in conquered lands, mixing with the local population, for lives based on farming.
In the far west, viking explorers reached Iceland and even north America and in the south, some raiders briefly terrorized the western Mediterranean. Migrating vikings settled in Scotland, England and Iceland, founded the city of Dublin and a jarl named Göngu Hrólfr a.k.a. Rollo took over Normandy. Eventually these Normans conquered England under William the Bastard and Sicily under count Roger. But by that time they mixed so much with conquered peoples, adopting their genes, languages, religion and culture, that they effectively ceased to be vikings, though their warrior spirit lived on for much longer.