This is just a stab at a counter argument but were the Vikings very successful? Today, apart from a few cold northern islands, nothing remains of anything truly Viking. They very quickly adopted the culture of any land they settled and their armies were often defeated once their opponents were properly organised. They might have killed the rulers of Northern England but they had nothing to replace them with. Even Canute left the English Witan to govern the Country. There was no unity in the term Viking as both opposing forces often contained warriors from the Scandinavian lands. Wulfstan, the English Archbishop of supposedly Viking York (Jorvik), used everyone, northern English and Viking, in his personal feud with the southern English. Recently in Dorset, UK a mass grave was found that contained fifty-four bodies. They had been tossed into the grave after their heads had been cut off. Tests on their teeth show they came from Scandinavia between AD 910 and AD 1030 and they were all in their early twenties. The Vikings never had it all their own way. Their fearsome reputation comes from literate Churchmen, as it was their Monasteries the Vikings sacked. When Harold fought William in 1066, after Harold had defeated the Norwegians, both men had Viking ancestry but one considered he was English the other nominally French. I believe the Normans owed their success to the quality of their horses. English Kings were constantly trying to improve the standard of their horses but normally they were only strong enough for transportation, e.g. to and from the battle. Normandy was ransacked barely one hundred years after 1066 and stopped being a major power. Now if the question was were the Vikings successful pirates I would have to agree.Incidentally were there actually real berserkers. They appear in the sagas and poems doing magical things, like the valkyries of myth, but are there any proofs that they actually existed.