Sussex (ˈsʌsɨks/), from the Old English 'Sūþsēaxe' ('South Saxons'), was a county in the south east of England. The foundation of the Kingdom of Sussex is recorded by the Anglo-Saxon Chronicle for the year AD 477, saying that Ælle arrived at a place called Cymenshore in three ships with his three sons and killed or put to flight the local inhabitants.

The foundation story is regarded as somewhat of a myth by most historians, although the archaeology suggests that Saxons did start to settle in the area in the late 5th century. The Kingdom of Sussex became the county of Sussex; then after the coming of Christianity; the see originally founded in Selsey, was moved to Chichester in the 11th century. The See of Chichester was coterminous with the county borders. In the 12th century the see was split into two archdeaconries centred at Chichester and Lewes.

After the Reform Act of 1832 Sussex was divided into the eastern division and the western division, these divisions were coterminous with the two archdeaconries of Chichester and Lewes. Sussex ceased to exist as a political entity in 1974, when, under an act of parliament, its eastern division became the county of East Sussex and the western division the county of West Sussex. There are several organisations that still operate within the ancient borders of Sussex although it is now the two counties of East and West Sussex, some examples being the Diocese of Chichester, Sussex Police and Sussex Archaeological Society.

Although the name Sussex is derived from the Saxon period between AD 477 to 1066, the history of human habitation in Sussex goes back to the Old Stone (Paleolithic) Age. Sussex has been occupied since those times and although it has been an industrious county it has succumbed to various persecutions, wars, invasions, political unrest and migrations throughout its long history.

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