Vikings
The Frankish Empire that came after them in Gaul became more and more aware of the northern threat. As the later Carolingian Empire expanded into northern Germany they came into contact with the Danish people. This is when the Vikings first entered into written history. The first recorded raid in the west was at Lindisfarne in 793. Why the Vikings suddenly began raiding is not completely clear. But a popular theory is the populations had grown to the point there was not enough food to feed everyone. The earliest raiders did not seem to want to move out of Scandinavia. They turned to looting, then returning home. These hit-and-run raids were made possible because the Vikings were master boat builders and they created flat bottomed boats ideal for journeys up rivers, where many monastic sites lay, ready for plundering.   This seems to indicate there was space enough for everyone. Because they found raiding so easy, it became more and more popular among the Vikings.