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In page Sussex:

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The foundation legend of Sussex is provided by the Anglo-Saxon Chronicle, which states that in the year AD 477 Ælle landed with his three sons.[1][2] Having fought on the banks of the Mearcredesburna,[3] it seems Ælle secured the area between the Ouse and Cuckmere in a treaty.[4] Traditionally this is thought to have been against native Britons, but it may have been to secure the area east of the Saxon Shore fort of Anderida from the influence of the Kingdom of Kent, with whom the South Saxons may have had occasional disputes.[5] Ælle was recognised as the first 'Bretwalda' or overlord of southern Britain. He was probably the most senior of the Anglo-Saxon kings and led the ill-fated campaign against King Arthur at Mount Badon.[citation needed]