Kent and Sussex
Offa appears to have exploited an unstable situation in Kent after 762. Kent had a long tradition of joint kingship, with east and west Kent under separate kings, though one king was typically dominant. Prior to 762 Kent was ruled by Æthelberht II and Eadberht I; Eadberht's son Eardwulf is also recorded as a king. Æthelberht died in 762, and Eadberht and Eardwulf are last mentioned in that same year. Charters from the next two years mention other kings of Kent, including Sigered, Eanmund and Heahberht. In 764, Offa granted land at Rochester in his own name, with Heahberht on the witness list as king of Kent. Another king of Kent, Egbert, appears on a charter in 765 along with Heahberht; the charter was subsequently confirmed by Offa. Offa's influence in Kent at this time is clear, and it has been suggested that Heahberht was installed by Offa as his client. There is less agreement among historians on whether Offa had general overlordship of Kent thereafter. He is known to have revoked a charter of Egbert's on the grounds that "it was wrong that his thegn should have presumed to give land allotted to him by his lord into the power of another without his witness", but the date of Egbert's original grant is unknown, as is the date of Offa's revocation of it. It may be that Offa was the effective overlord of Kent from 764 until at least 776. The limited evidence for Offa's direct involvement in the kingdom between 765 and 776 includes two charters of 774 in which he grants land in Kent; but there are doubts about their authenticity, so Offa's intervention in Kent prior to 776 may have been limited to the years 764–765.
The Anglo-Saxon Chronicle records that "the Mercians and the inhabitants of Kent fought at Otford" in 776, but does not give the outcome of the battle. It has traditionally been interpreted as a Mercian victory, but there is no evidence for Offa's authority over Kent until 785: a charter from 784 mentions only a Kentish king named Ealhmund, which may indicate that the Mercians were in fact defeated at Otford. The cause of the conflict is also unknown: if Offa was ruling Kent before 776, the battle of Otford was probably a rebellion against Mercian control. However, Ealhmund does not appear again in the historical record, and a sequence of charters by Offa from the years 785–789 makes his authority clear. During these years he treated Kent "as an ordinary province of the Mercian kingdom", and his actions have been seen as going beyond the normal relation of overlordship and extending to the annexation of Kent and the elimination of a local royal line. After 785, in the words of one historian, "Offa was the rival, not the overlord, of Kentish kings". Mercian control lasted until 796, the year of Offa's death, when Eadberht Praen was temporarily successful in regaining Kentish independence.
Ealhmund was probably the father of Egbert of Wessex, and it is possible that Offa's interventions in Kent in the mid-780s are connected to the subsequent exile of Egbert to Francia. The Chronicle claims that when Egbert invaded Kent in 825, the men of the southeast turned to him "because earlier they were wrongly forced away from his relatives". This is likely to be an allusion to Ealhmund, and may imply that Ealhmund had a local overlordship of the southeastern kingdoms. If so, Offa's intervention was probably intended to gain control of this relationship and take over the dominance of the associated kingdoms.
The evidence for Offa's involvement in the kingdom of Sussex comes from charters, and as with Kent there is no clear consensus among historians on the course of events. What little evidence survives that bears on Sussex's kings indicates that several kings ruled at once, and it may never have formed a single kingdom. It has been argued that Offa's authority was recognised early in his reign by local kings in western Sussex, but that eastern Sussex (the area around Hastings) submitted to him less readily. Simeon of Durham, a twelfth-century chronicler, records that in 771 Offa defeated "the people of Hastings", which may record the extension of Offa's dominion over the entire kingdom. However, doubts have been expressed about the authenticity of the charters which support this version of events, and it is possible that Offa's direct involvement in Sussex was limited to a short period around 770–771. After 772, there is no further evidence of Mercian involvement in Sussex until c. 790, and it may be that Offa gained control of Sussex in the late 780s, as he did in Kent.
Read more about this topic: Offa Of Mercia