The Great Northern War was the name given by the Romans to a series of conflicts they fought against the Scandinavians, while trying to conquer that area. At first, the Romans fought against Swedish and Norwegian norsemen; but after years of combat, the Swedish agreed to surrender to Rome, and become a 'client state' of the Empire. The Norwegians continued the fight alone for a while, and managed to hold off because their home terrain seemed impenetrable to the Romans, who were not used to such a Climate.
Meanwhile, the Norse were conducting their Viking Raids against several Roman settlements in Western Europe; and there was an outcry of Provincial Governors to be better armed against this new threat. The Roman army ignored these requests; and instead focused on trying to defeat the Norse in Norway, thereby stopping the Viking raids.