Finer grained standards that are part of this one
The Viking attacks took place mostly in the 9th and 10th centuries.
Tribal units were led by chieftains.
Lack of arable land led to exploration and invasion.
Invasions disrupted trade; towns declined.
The Vikings observed a polytheistic religion, but gradually converted to Christianity.
Viking attacks contributed to the collapse of the Frankish Empire founded by Charlemagne.
Vikings settled in the areas known today as Russia, Iceland, and Greenland, and briefly in North America.
Eastern European sea and river trade.