It is unlikely that Offa had significant influence in the early years of his reign outside the traditional Mercian heartland.
The overlordship of the southern English which had been exerted by Æthelbald appears to have collapsed during the civil strife over the succession, and it is not until 764, when evidence emerges of Offa's influence in Kent, that Mercian power can be seen expanding again.
Charters were documents which granted land to followers or to churchmen and were witnessed by the kings who had the authority to grant the land.
A charter might record the names of both a subject king and his overlord on the witness list appended to the grant.
According to a later continuation of Bede's Historia Ecclesiastica (written anonymously after Bede's death) the king was "treacherously murdered at night by his own bodyguards," though the reason why is unrecorded.
Æthelbald was initially succeeded by Beornred, about whom little is known.
A significant corpus of letters dates from the period, especially from Alcuin, an English deacon and scholar who spent over a decade at Charlemagne's court as one of his chief advisors, and corresponded with kings, nobles and ecclesiastics throughout England.
Offa's ancestry is given in the Anglian collection, a set of genealogies that include lines of descent for four Mercian kings.Offa persuaded Pope Adrian I to divide the archdiocese of Canterbury in two, creating a new archdiocese of Lichfield.This reduction in the power of Canterbury may have been motivated by Offa's desire to have an archbishop consecrate his son Ecgfrith as king, since it is possible Jaenberht refused to perform the ceremony, which took place in 787.In one charter Offa refers to Æthelbald as his kinsman, and Headbert, Æthelbald's brother, continued to witness charters after Offa rose to power.Æthelbald, who had ruled Mercia since 716, was assassinated in 757.He also became the overlord of East Anglia and had King Æthelberht II of East Anglia beheaded in 794, perhaps for rebelling against him.