North Germanic peoples
Vikings were also active in Francia. Some of these Vikings would sail through the Strait of Gibraltar and pillage the coasts of the Mediterranean Sea. In the early 10th century, a group of Vikings under the leadership of Rollo settled in Rouen, France, and established the Duchy of Normandy. The descendants of these Vikings, known as the "Normans", would in the 11th century conquer England, Southern Italy, and North Africa, and play a leading role in launching the Crusades. Sub-groups of the Normans include Anglo-Normans, Scoto-Normans, Cambro-Normans, Hiberno-Normans and Italo-Normans.
Naval warfare
In the 8th century the Vikings appeared, although their usual style was to appear quickly, plunder, and disappear, preferably attacking undefended locations. The Vikings raided places along the coastline of England and France, with the greatest threats being in England. They would raid monasteries for their wealth and lack of formidable defenders. They also utilized rivers and other auxiliary waterways to work their way inland in the eventual invasion of Britain. They wreaked havoc in Northumbria and Mercia and the rest of Anglia before being halted by Wessex. King Alfred the Great of England was able to stay the Viking invasions with a pivotal victory at the Battle of Edington. Alfred defeated Guthrum, establishing the boundaries of Danelaw in an 884 treaty. The effectiveness of Alfred's 'fleet' has been debated; Dr. Kenneth Harl has pointed out that as few as eleven ships were sent to combat the Vikings, only two of which were not beaten back or captured.