Sussex, kingdom of, one of the Anglo-Saxon heptarchy (seven kingdoms) in England, located S of the Weald. It was settled in the late 5th cent. (according to tradition in 477) by Saxons under Ælle, who defeated the Celts in several battles and established a brief military supremacy. Little is known of the kingdom's history for almost two centuries. The South Saxons remained heathen until St. Wilfrid, bishop of York, led (681–86) the Christian conversion of the people. Conquered (685–88) by Cædwalla of Wessex, Sussex remained subject to his successor, Ine (688–726). By 771, Offa of Mercia had conquered all the marginal kingdoms (including Sussex) into which the South Saxons were divided. They remained under Mercia until joined with other eastern states in submitting to Egbert of Wessex in 825.
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