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In page Angles (tribe):

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According to sources such as the History of Bede, after the invasion of Britannia, the Angles split up and founded the kingdoms of Northumbria, East Anglia, and Mercia. H. R. Loyn has observed in this context that "a sea voyage is perilous to tribal institutions",[1] and the apparently tribe-based kingdoms were formed in England. Early times had two northern kingdoms (Bernicia and Deira) and two midland ones (Middle Anglia and Mercia), which had by the seventh century resolved themselves into two Angle kingdoms, viz., Northumbria and Mercia.[citation needed]