Why were they so powerful?

Well we must ask us in this point what did make the Vikings so successful and powerful men? Nobody could stop them and everyone feared them.
                                       

The answer could be obvious if you notice that they had got the most fast, resistant and best prepared ships to sail of the period. Their vessels or Drakkars that were longships, narrow and very flexible vessels were used by the them for trade, exploration, and warfare. They were also called "dragonships" because they often had a dragon-shaped. The Longships had been the symbol of the Vikings' naval power for a long time. They were expert sailors,  they didn´t even need ship's compass in order to orient themselves, they just took advantage of the position of the Sun, Moon or stars to get orientation or symply they sailed alonside the coast but not only they used the sea, they used the rivers as well, thanks to this, they got into all parts of Europe. Their vessels were not too much big and they could got inside any river, spreading out like a plague throughout Europe. One remarkable thing is that they had to put up with storms, torrential rains while sailing because of these vessels were completely open. Probably many Vikings would died from hypothermia and others things associated with the cold, just only imagine that dark nights throughout open sea during winter, with strong storms etc you can realice how terrible had to be and how powerful were these men.
                                           
Another important reason was their cruelty and wickedness bigger than the rest of the tribes of Europe at that particular time, they were plunderers and killers. Actually we can say that the Viking were warriors, fierce warriors without compassion of any kind, they killed everyone, even babies, children and women, although women alao used to be sold by the Vikings after being raped for the Vikings. Their most important weapon was the axe, they had a great skill using it, they threw them against their enemies with such accuracy that mostly the enemy died with the impact or got disabled.
                                                         

One easy target and too much desirable for them were the churches and monasteries, because of the monks didn´t put up any ``resistance´´ and they had so much silver inside them. For the Vikings silver was their more desirable treasure like gold for Spanish in the 15th century but actually they didn´t want these treasures although it was very important for them so these were a mean of exchange.

Ambition and greediness were other factors for the Viking´s conquest, the more they had got, the more they would want to. The conquests and plunders became in a current activity for them like commerce. These Viking warriors didn´t like work for a living, they prefered robbing.