South Saxons were the followers of King Ælle a warlord from Old Saxony in north-western Germany who were among the Anglo-Saxon Dark Age invaders of Britannia at the end of the 5th Century. The name "South Saxons" is probably not a self-appellation but rather a geographical distinction made later by scholars in Canterbury in Kent for the southernmost Saxon polity of Dark Age Britain. South Saxon in Old English is Suð Seax from which we get Sussex. The terms "South Saxons" and "Kingdom of Sussex" are interchangeable.
The South Saxon kingdom was founded after the final collapse of the Romano-British defence of the Civitas Regnensis in 491 when the Saxon Shore fort of Anderitum (Pevensey Castle) fell and it's garrison was slaughtered. The campaign of conquest led by Ælle had begun in 477 when he first made landing at Cymenes ora (somewhere in modern Sussex) with his sons and slew the local Welsh defenders and drove the remainder into the Forest of Andred. A subsequent and decisive battle between Ælle's forces and the native inhabitants is recorded by the Anglo-Saxon Chronicle at Mearcredes burne in 485 before the siege and capture of Caer Andred in 491.
The dynasty established by King Ælle - the Ællingus - ruled with varying degrees of independence at least until 825 when the kingdom was annexed by Wessex (the West Saxons). At some points Sussex was united under one king and at other times the kingdom appears to have been divided between several 'sub-kings' but all were probably from the same ruling clan and could trace their ancestry back to Ælle.
Between 491 and 607 there are no known records of the South Saxons apart from that they existed. In 607 it is recorded in the Anglo-Saxon Chronicle that there was war between Sussex and Wessex and this may have been over sovereignty of land to the west called The Meon opposite the Isle of Wight or perhaps over sovereignty of Surrey to the north - this is however, just speculation. In 681 the Chronicle records that Saint Wilfrid of York was exiled in Sussex from his home in Northumbria where he remained until 686 converting its pagan inhabitants. According to Bede, Aethelwalh, king of Sussex, had been previously baptised. After Saint Wilfrid's apparent miracles in relieving a famine which occurred in Sussex the king granted him eighty-seven hides (an area of land) in and near the peninsula of Selsey which remained thereafter the seat of the South Saxon bishopric (and thus also their capital) until the Norman Conquest.
Shortly afterwards, however, Aethelwalh was slain and his kingdom ravaged by the exiled West Saxon prince Caedwalla. The latter was eventually expelled by two princes named Berhthun and Andhun, who thereupon assumed the government of the kingdom. In 686 the South Saxons intervened in a civil war in Kent, in support of a Prince Eadric, but soon afterwards King Berhthun was killed and the kingdom subjugated for a time by Caedwalla, who had by now become king of Wessex.
In 692 a grant is made by a king called Nothelm of Sussex to his sister, which is witnessed by two other "kings" called Nunna and Uuattus. Nunna is probably Nun, a relative of King Ine of Wessex who fought with him against the West Welsh, in 710. In 722 we find Ine of Wessex at war with the South Saxons, apparently because they were supporting a rival claimant of the West Saxon throne.
After this we hear nothing more until about 765, when a grant of land is made by a king named Aldwulf. In 770 a grant is made by a King Osmund of Sussex which is witnessed by King Offa of Mercia. Offa also appears as witness to two charters of an Æthelbert of Sussex in 772. It is probable that about this time Offa claimed overlordship of the kingdom of Sussex, as several local rulers, Osmund, Ælfwald and Oslac, who had previously used the royal title, now sign with that of "dux".
Mercian power collapsed in the years following Offa's death in 796 and the South Saxons re-emerged as an independent political entity. But this was to be short lived indeed because a new power was rising - that of Wessex - and Sussex was to become the first kingdom of the old heptarchy to be annexed by Wessex in a process which was to bring about the gradual unification of the Ænglecynne and the foundation of a united England. Sussex was annexed by King Egbert of Wessex in 825 and from this time onwards they remained subject to the West Saxon dynasty.
It is thought that the Æellingi (the South Saxon royal house) continued to govern Sussex as eorldermen (earls) under West Saxon sovereignty until the Norman Conquest in 1066.
Stephens ,The south Saxon diocese, Selsey-Chichester, 1881
Bede, Ecclesiastical History of the English People, 731 AD, Translation Leo Sherley-Price 1955 Stenton, F M Anglo-Saxon England (1971).