The realm of the South Saxons, Sussex was a small kingdom on the English Channel, including the South Downs and the Weald. It stretched west from Kent to about Portsmouth. The Anglo-Saxon chronicle for the year 477 states:
Her cuom Ælle on Bretenlond & his .iii. suna, Cymen & Wlencing & Cissa, mid .iii. scipum on þa stowe þe is nemned Cymenesora, & þær ofslogon monige Wealas & sume on fleame bedrifon on þone wudu þe is genemned Andredesleage.
"This year came Ælle to Britain, with his three sons,
Cymen, and Wlenking, and Cissa, in three ships; landing at a
place that is called Cymenshore. There they slew many of the
Welsh; and some in flight they drove into the wood that is called
Anglo-saxon rulers of Sussex
Sussex essentially became a part of Wessex after this point. It would often be given to supernumerary sons along with Surrey and Kent. Rulers of Sussex continue in the nodes for Kent and Wessex, and later Rulers of England.
For the next 1200 years or so, Sussex was a shire in England. But now, Sussex has been split into two counties, East Sussex and West Sussex.