In the history of Great Britain, Anglo-Saxon England refers to the historical land roughly corresponding to present-day England, as it existed from the 5th to the 11th century, but not including Devon and Cornwall until the 9th century.
The Anglo-Saxons were the members of Germanic-speaking groups who migrated to the southern half of the island from continental Europe, and their cultural descendants. Anglo-Saxon history thus begins during the period of Sub-Roman Britain following the end of Roman control, and traces the establishment of Anglo-Saxon kingdoms in the 5th and 6th centuries (conventionally identified as seven main kingdoms: Northumbria, Mercia, East Anglia, Essex, Kent, Sussex, and Wessex), their Christianisation during the 7th century, the threat of Viking invasions and Danish settlers, the gradual unification of England under Wessex hegemony during the 9th and 10th centuries, and ending with the Norman conquest of England by William the Conqueror in 1066. Anglo-Saxon identity survived beyond the Norman Conquest,[1] and came to be known as Englishry under Norman rule and ultimately developed into the modern English people.
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Isn't Wikipedia wonderful?
Best to say, in very broad terms, "Germanic" in that the separate territories occupied by the Angles and the Saxons (and the Jutes, etc) before they started to invade England bear very little resemblance to the borders of Germany today.
And remember that Germany became a unified country only in 1871, and since 1918 it has had three substantial changes of boundary. In the Middle Ages much of the Germanic lands (within various borders at various times) formed the heart of the "Holy Roman Empire", with no connection to the pope in Rome, and often in dispute or actual conflict with the Pope.
The more you look ay European history, the more you will see that peoples have moved from place to place in the last 2,000 years and more. There are few countries in Europe with borders more than 120 years old. Since 1990 17 new countries have been created in Europe or on its edges.
Even the USA did not fully sort out its border with Canada until 1903 (Alaska Panhandle).
The problem you are having is that you are thinking of the Anglo-Saxons as one people.
That was not the case, not only did Angles, Saxons and Jutes have different languages and customs, even within the groups there were cultural differences, the Saxons of Wessex, for example, were not the same as the Saxons of Essex, and certainly not the same as the Angles of Mercia or Jutes of Kent, there were some basic similarities but also evident differences.
England as a vague national description could not be usefully applied until centuries after their arrivals, and almost immediately separated again.
Try F M Stenton's Anglo-Saxon England.
The Angles, Saxons and Jutes came from Germany, Denmark, and Norway. They migrated to Britain in the 5th and 6th centuries AD after the Romans had left. They didn't invade or conquer England, they simply emigrated in family or tribal groups and quietly settled in the country eventually becoming the dominant ethnic group,