Sussex, i.e. the kingdom of the South Saxons, approximated to the area of the modern counties of East and West Sussex. Its history is but dimly recorded. In the late-8th century Sussex was annexed by Mercia. Following Mercia’s decisive defeat by Wessex, in 825, Sussex submitted to, and was eventually absorbed into, Wessex.
The Anglo-Saxon Chronicle, s.a. 477, notes that: “In this year Ælle came to Britain, and his 3 sons, Cymen and Wlencing and Cissa, with 3 ships, at a place which is named Cymenesora, and there slew many Welsh [i.e. Britons], and drove some in flight into the wood that is called Andredeslea [the Weald].[*]” An entry for 485 reports Ælle in battle against the Britons “near the bank of Mearcredesburna [unidentified]”. Presumably Ælle was the victor – the Chronicle doesn’t say. There is no doubt about the outcome in 491, though, when: “Ælle and Cissa besieged Andredesceaster [the Roman fort of Anderida, Pevensey], and slew all that dwelt therein; not even one Briton was there left.” Ælle’s authority was apparently not just local. He is described by Bede (HE II, 5) as the first “of the English kings who ruled over all the southern provinces that are divided from the northern by the river Humber and the borders contiguous to it”. Taking its cue from Bede, the Anglo-Saxon Chronicle lists Ælle as the first Bretwalda. Neither Bede nor the Chronicle make further mention of Ælle or his sons.
Henry of Huntingdon apparently used his imagination to flesh-out the bare-bones account of Ælle’s activities provided by the Anglo-Saxon Chronicle.[*] Perhaps he was also using his imagination when he averred that: “Around this time  the death occurred of Ælle, king of the South Saxons, who had held all the powers of kingship over the English, in that he held authority over kings, nobles and commanders. After him reigned his son Cissa and their descendants after them” (HA II, 15).
Henry of Huntingdon is the earliest source to claim that Cissa succeeded Ælle. Roger of Wendover, who based his account of Ælle’s deeds on Henry’s story, records Cissa’s accession: “In the year of grace 514 … died Ælle, whom all the Saxons acknowledged as their king. He was succeeded by his son Cissa, from whom Chichester, which he founded, received its name.”[*] Roger, however, goes on to assert that Cissa was king of Sussex in 586, and says: “In the year of grace 590, on the death of Cissa, king of the South Saxons, that kingdom devolved on Ceawlin, king of the West Saxons.” Clearly, a reign of seventy-six years is not credible, and, since Cissa was apparently of fighting-age when he arrived in Britain with his father, he would have been something like one hundred and thirty years-old.
Anglo-Saxon Chronicle s.a. 607: “In this year Ceolwulf fought against the South Saxons.”
… he [Wilfrid] could not be restrained from the ministry of the Gospel; for, taking his way into the province of the South Saxons, which extends from Kent to the south and west, as far as the West Saxons, containing land of 7 thousand families, and was at that time still in bondage to pagan rites, he administered to them the Word of faith, and the Baptism of salvation. Æthelwalh, king of that nation, had been, not long before, baptized in the province of the Mercians, at the instance of King Wulfhere, who was present, and received him as his godson when he came forth from the font, and in token of this adoption gave him two provinces, to wit, the Isle of Wight, and the province of the Meonware, in the country of the West Saxons.[*] The bishop, therefore, with the king’s consent, or rather to his great joy, cleansed in the sacred font the foremost ealdormen and thegns of that country; and the priests, Eappa and Padda, and Burghelm and Oiddi, either then, or afterwards, baptized the rest of the people. The queen, whose name was Eafe, had been baptized in her own country, the province of the Hwicce.[*] She was the daughter of Eanfrith, the brother of Eanhere, who were both Christians, as were their people; but all the province of the South Saxons was ignorant of the Name of God and the faith.[*] But there was among them a certain monk of the Scottish nation [i.e. an Irish monk], whose name was Dicul, who had a very small monastery, at the place called Bosanhamm [Bosham], encompassed by woods and seas, and in it there were 5 or 6 brothers, who served the Lord in humility and poverty; but none of the natives cared either to follow their course of life, or hear their preaching.
But Bishop Wilfrid, while preaching the Gospel to the people, not only delivered them from the misery of eternal damnation, but also from a terrible calamity of temporal death. For no rain had fallen in that district for three years before his arrival in the province, whereupon a grievous famine fell upon the people and pitilessly destroyed them; insomuch that it is said that often 40 or 50 men, wasted with hunger, would go together to some precipice, or to the sea-shore, and there, hand in hand, in piteous wise cast them themselves down either to perish by the fall, or be swallowed up by the waves. But on the very day on which the nation received the Baptism of the faith, there fell a soft but plentiful rain; the earth revived, the fields grew green again, and the season was pleasant and fruitful. Thus the old superstition was cast away, and idolatry renounced, the heart and flesh of all rejoiced in the living God, for they perceived that He who is the true God had enriched them by His heavenly grace with both inward and outward blessings. For the bishop, when he came into the province, and found so great misery from famine there, taught them to get their food by fishing; for their sea and rivers abounded in fish, but the people had no skill to take any of them, except eels alone. The bishop’s men having gathered eel-nets everywhere, cast them into the sea, and by the blessing of God took 300 fishes of divers sorts, which being divided into three parts, they gave a hundred to the poor, a hundred to those of whom they had the nets, and kept a hundred for their own use. By this benefit the bishop gained the affections of them all, and they began more readily at his preaching to hope for heavenly blessings, seeing that by his help they had received those which are temporal.
At this time, King Æthelwalh gave to the most reverend prelate, Wilfrid, land to the extent of 87 families, to maintain his company who were wandering in exile. The place is called Selæseu [Selsey], that is, the Island of the Sea-calf [i.e. Seal]; it is encompassed by the sea on all sides, except the west, where is an entrance about the cast of a sling in width; which sort of place is by the Latins called a peninsula, by the Greeks, a cherronesos. Bishop Wilfrid, having this place given him, founded therein a monastery, chiefly of the brethren he had brought with him, and established a rule of life; and his successors are known to be there to this day. He himself, both in word and deed performed the duties of a bishop in those parts during the space of 5 years, until the death of King Ecgfrith, and was justly honoured by all. And forasmuch as the king, together with the said place, gave him all the goods that were therein, with the lands and men, he instructed all the people in the faith of Christ, and cleansed them in the water of Baptism. Among whom were two hundred and fifty male and female slaves, all of whom he saved by Baptism from slavery to the Devil, and in like manner, by giving them their liberty, set them free from slavery to man.HE IV, 13
At some stage between 681 and 685:
… Cædwalla, a young man of great vigour, of the royal race of the Gewisse [West Saxons], an exile from his country, came with an army, slew Æthelwalh, and wasted that province with cruel slaughter and devastation; but he was soon expelled by Berhthun and Andhun, the king’s ealdormen, who afterwards held the government of the province.HE IV, 15
It isn't clear, therefore, whether Æthelwalh was still king when, in February 685, the South Saxons helped Eadric of Kent overthrow his uncle, Hlothere.
Presumably Berhthun and Andhun ruled as joint kings after Æthelwalh’s death.
In 685/6 Cædwalla established himself as king of the West Saxons. The Anglo-Saxon Chronicle reports that, in 686, he: “ravaged Kent and Wight.” The Isle of Wight and the province of the Meonware, on the mainland opposite, had been subject to the South Saxons since Wulfhere had gifted them to Æthelwalh. Bede writes: “After Cædwalla had obtained possession of the kingdom of the Gewisse, he took also the Isle of Wight, which till then was entirely given over to idolatry, and by merciless slaughter endeavoured to destroy all the inhabitants thereof, and to place in their stead people from his own province” (HE IV, 16). The Isle of Wight still had its own ruling dynasty, which was ruthlessly suppressed by Cædwalla. Presumably he had already taken the mainland province before launching his assault on the Isle of Wight. Around this time, Cædwalla, once again, attacked the South Saxons (Bede just says it was “when he [Cædwalla] was king of the Gewisse”). Berhthun was killed (nothing is known of Andhun’s fate), and Sussex: “was reduced to more grievous slavery. Ine, likewise, who reigned after Cædwalla,[*] oppressed that country with the like servitude for many years; for which reason, during all that time, they could have no bishop of their own; but their first bishop, Wilfrid, having been recalled home,[*] they were subject to the bishop of the Gewisse, that is, the West Saxons, who were in the city of Venta [Winchester].” (HE IV, 15). Between 705 and 709, the South Saxons got their own bishop – Eadberht, abbot of Selsey, being first to fill the position.[*]
After the brief burst of illumination provided by Bede, the history of Sussex, once more, fades into almost complete obscurity. The few surviving charters show that Sussex was divided amongst a number of kings. The name of King Nunna (also called Nothhelm) is found associated with a king called Watt in 692 (S45), and a king and queen called Æthelstan and Æthelthryth in 714 (S42). The Anglo-Saxon Chronicle refers to Nunna as a “kinsman” of, the above mentioned West Saxon king, Ine, when it reports that, in 710, the two of them “fought against Geraint, king of the Welsh [i.e. the Britons of Dumnonia[*]]”. In 722, however: “Ine fought against the South Saxons.” The South Saxons had given refuge to one Ealdberht, whom Ine had exiled. Ine attacked Sussex again in 725, this time killing Ealdberht. Ine’s grip on power was slipping. He abdicated in 726, and subsequently all English kingdoms south of the Humber became subject to Æthelbald, king of Mercia – this had been achieved by 731, when Bede finished his ‘Ecclesiastical History’. In 757, Æthelbald was murdered, and Mercia underwent a period of instability as Offa established his authority.
Florence of Worcester, s.a. 758, reports that “at this period” Osmund was king of the South Saxons. There were, though, three other South Saxon kings who ruled simultaneously with Osmund. The three feature together in an undated charter (S50) – Ealdwulf, whose charter it is, and, as witnesses, Ælfwald and Oslac (?). The names of Offa, king of Mercia, his wife and son were subsequently added to the witness-list.[*] In a charter of, probably, 765 (S48), Osmund had made a grant of land as his own man, but in 770 he apparently required the consent of Offa (S49).[*] Symeon of Durham (HR) records that, in 771, Offa “subdued by arms the people of the Hestingi [who gave their name to Hastings[*]].” In 772, Offa made a grant of land in Bexhill to Oswald, bishop of the South Saxons (S108). The charter was witnessed by Osmund, Ælfwald and Oslac, but now, instead of being titled rex (king), they are titled dux. There is, therefore, a reasonable likelihood that, by 772, Offa had annexed Sussex.[*]
Following the defeat of Beornwulf of Mercia, by Egbert of Wessex, in 825, and the subsequent expulsion of the incumbent ruler of Kent, one Baldred, by Egbert’s son, Æthelwulf; Kent, Sussex, Essex, and also Surrey, surrendered to Wessex. These provinces formed a sub-kingdom of Wessex, and then, in 860, were integrated into Wessex proper.