Learnings: Scouts Before the Storm
According to the 789 annal of the Anglo-Saxon Chronicle seen here, the first Viking raid on Anglo-Saxon soil took place many decades before the advent of the "Great Heathen Army". Vikings scouts would travel to a region, observing and trading with locals. This was done to gain insights not only on the land itself, but its power structures and politics. The Vikings used this knowledge to decide whether they should attack an Anglo-Saxon leader or serve him as a mercenary. They also used this intelligence to plan their eventual invasion.
Viking scouts were not always subtle in their spying. Sometimes they left a trail of burned houses and villages. But this was no accident. Vikings sought to intimidate their rivals through violence and destruction. They were fighting a psychological, as well as a physical, war.