One widely held idea is that it was a quest for retaliation against continental Europeans for their previous invasions of Viking homelands, such as Charlemagne's campaign to force Scandinavian pagans to convert to Christianity by killing any who refused to become baptized.[2][3][4][5][6] The historian Rudolf Simek has observed, "It is not a coincidence if the early Viking activity occurred during the reign of Charlemagne."[7][8] Those who favor this explanation point out that the penetration of Christianity into Scandinavia caused serious conflict and divided Norway for almost a century.[9] However, the first target of Viking raids was not the Frankish Kingdom, but Christian monasteries in England.