After finally subduing the Saracen threat from the east, the eighth century brought to Europe a new invader--this time, unexpectedly from the north. These Scandinavians seemed but harmless savages conducting minor raids in the beginning, but they would soon wreak havoc throughout the Carolingian civilization. Although the main cause of their first attacks on western Europe are not known, from the start of the ninth century these Viking raids were already spreading so quickly that there was no immediate end in sight (by the mid-ninth century, it was estimated that the Viking Rorik had under his command 600 ships raiding on the Elbe river alone (Bradbury 21)).
Because, at first, the main objective of the Viking raids was mere plunder, the original targets of their melee were the easier targets of monasteries, trading outposts and any other lightly guarded target of movable wealth (Cambridge U 18-19). It was the objective of plunder and the absence of a desire to siege and acquire territory that lent the Viking strategy a path to success.
Their raiding practices, in the beginning, consisted of the rapid deployment of 30-50 men aboard their longships--with a design that made it possible to travel the deep seas as well as the shallow rivers that ran deep into the European mainland.
They would later combine the expert use of their longships, which already made it near impossible for contemporary Carolingian armies to track (also the randomness of the raids in such a vast area made any predictions obsolete), with a full swing transformation into the use of horses. At that point, they would use the ships to transport their armies and its supplies to a certain region, and then with supplies close at hand, the now horse-mobile Vikings could strike even faster at river/sea-side cities, and strike further...