In the first half of the eighth century, the dominant Anglo-Saxon ruler was King Æthelbald of Mercia, who by 731 had become the overlord of all the provinces south of the river Humber.

Æthelbald was one of a number of strong Mercian kings who ruled from the mid-seventh century to the early ninth, and it was not until the reign of Egbert of Wessex in the ninth century that Mercian power began to wane.

Æthelbald, who ruled Mercia for most of the forty years before Offa, was also descended from Eowa according to the genealogies: Offa's grandfather, Eanwulf, was Æthelbald's second cousin.

Æthelbald, who had ruled Mercia since 716, was assassinated in 757.

The marriage to a Frankish princess who had her own royal lineage could have produced heirs more throne-worthy than Æthelbald.

Asser, who was hostile to Æthelbald both because of his revolt against his father and because of his uncanonical marriage, described him as " iniquitous and grasping ", and his reign " two and a half lawless years ".

An early source, Felix's Life of Saint Guthlac, reveals that it was Ceolred who drove Æthelbald into exile.

With the exception of the short reign of Beornrad, who succeeded Æthelbald for less than a year, Mercia was ruled for eighty years by two of the most powerful Anglo-Saxon kings, Æthelbald and Offa.

There is little direct evidence of the relationship between Æthelbald and the kings who were dependent on him.

Hence it may be that Æthelbald helped establish both Aethelheard and his brother, Cuthred, who succeeded Aethelheard in 739.

The lack of evidence should not obscure the fact that Bede, who was after all a contemporary chronicler, summarized the situation of England in 731 by listing the bishops in office in southern England, and adding that " all these provinces, together with the others south of the river Humber and their kings, are subject to Æthelbald, King of the Mercians.

One suggested interpretation is that since Bede was writing during Æthelbald's reign, the original seven he listed were essentially those kings who could be seen as prototypes of Æthelbald in their domination of England south of the Humber.

Æthelbald, who came to the throne in 716, had established himself as the overlord of the southern Anglo-Saxons by 731.

The author, Felix, reports conflicts with the Britons: " in the days of Coenred King of the Mercians, [...] the Britons the implacable enemies of the Saxon race, were troubling the English with their attacks, their pillaging, and their devastations of the people [...]" To counter such attacks, Æthelbald, who came to the throne in 716, was once thought to have built Wat's Dyke, an earthwork barrier in northern Wales ; but this now seems unlikely, since an excavation of the Dyke in 1997 found charcoal from a hearth which was radiocarbon-dated to some time between 411 and 561.

Æthelbald, a member of another branch of the Mercian royal line who had been forced into exile during Ceolred's rule, succeeded him.

Beorhtwulf, who succeeded to the throne that year, is likely to have come from the B group, which may also have included the ill-fated Beornred who " held a little while and unhappily " after the murder of King Æthelbald in 757.

Dying without issue Æthelbald was followed by his brother Æthelberht who left two children who were young and skipped.

He was succeeded by Æthelbald, who married Judith, his step-mother, probably to enhance his status because she was the daughter of the Holy Roman Emperor.

: Once King Æthelwulf was dead, Æthelbald, his son, against God's prohibition and Christian dignity, and also contrary to the practice of all pagans, took over his father's marriage-bed and married Judith, daughter of Charles, king of the Franks, incurring great disgrace from all who heard of it.

He gave sanctuary to Æthelbald, future king of Mercia, who was fleeing from his cousin Ceolred.

Whether or not he owed his appointment to Æthelbald, Nothhelm was one of a number of Mercians who became Archbishop of Canterbury during the 730s and 740s, during a time of expanding Mercian influence.

Æthelbald suffered a setback in 752, when he was defeated by the West Saxons under Cuthred, but he seems to have restored his supremacy over Wessex by 757.

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.

According to Alfred the Great's biographer, Asser, during Æthelwulf's absence there may have been a plot hatched to prevent the king's return either by Æthelbald, or by Ealhstan, Bishop of Sherborne and Eanwulf, Ealdorman of Somerset, or by all three.

The subsequent 747 council of Clovesho, and a charter Æthelbald issued at Gumley in 749 — which freed the church from some of its obligations — may have been responses to Boniface's letter.

Generally, a king subject to an overlord such as Æthelbald would still be regarded as a king, but would have his independence curtailed in some respects.

Æthelbald seems to have reasserted his authority over the West Saxons by the time of his death, since a later West Saxon king, Cynewulf, is recorded as witnessing a charter of Æthelbald at the very beginning of his reign, in 757.

Though Boniface's letter praises Æthelbald's faith and alms-giving, its criticisms have strongly coloured subsequent opinion of Æthelbald.

Æthelbald may have influenced the appointment of successive archbishops of Canterbury in Tatwine, Nothelm, and Cuthbert, the latter probably the former bishop of Hereford ; and despite Boniface's strong criticisms, there is evidence of Æthelbald's positive interest in church affairs.

Æthelbald was buried at Repton, in a crypt which still can be seen ; a contemporary is reported to have seen a vision of him in hell, reinforcing the impression of a king not universally well-regarded.

Egbert appears to have been independent of Mercia from the beginning of his reign, and Wessex's independence meant that Coenwulf was never able to claim the overlordship of the southern English that had belonged to Offa and Æthelbald.

He may have come to power under the influence of Æthelbald of Mercia, since he was recorded as a witness to a charter of Æthelbald shortly thereafter.

The reduced prestige of both Coenred and his successor, Ceolred, may have stirred unrest among the Mercian nobility: Æthelbald was in exile during Ceolred's reign, and the survival of a hostile account of Ceolred may indicate a more general dissatisfaction with the ruling line.

The reduced prestige of both Ceolred and his predecessor, Coenred, may have stirred unrest among the Mercian nobility: Æthelbald was in exile during Ceolred's reign, and the survival of a hostile account of Ceolred may indicate a more general dissatisfaction with the ruling line.

It is possible that his success was due to the support of Æthelbald of Mercia, since he seems to have been subject to Æthelbald afterward.

However, Æthelheard's lack of independence does not seem to have prevented Æthelbald from taking considerable territory from Wessex in 733, including the royal manor of Somerton.

The Vikings were defeated by Æthelwulf and his sons, Æthelstan and Æthelbald, but the economic impact appears to have been significant, as Mercian coinage in London was very limited after 851.

The first church of Hoo St Werburgh may have been built in the reign of the 8th century King Æthelbald of Mercia, though presumably a monastery existed nearby at an earlier time.

Little is known about the history of the East Saxons during the eighth century, but what evidence there is indicates that both London and Middlesex, which had been part of the kingdom of Essex, were finally brought under Mercian control during the reign of Æthelbald.

The mounted figure on the Repton Stone in Derby Museum has been identified as Æthelbald.

A fragment of a cross shaft from Repton includes on one face a carved image of a mounted man which, it has been suggested, may be a memorial to Æthelbald.

Judith had previously been married to Æthelwulf and his son ( from an earlier marriage ) Æthelbald, kings of Wessex, but after the latter's death in 860 she had returned to France.

Earnwine's father had been an exile in the north after his defeat in the civil war of 705 – 706, and it may be that Óengus, or Æthelbald, or both, had tried to place him on the Northumbrian throne.

Unlike the straightforward narrative of the attacks on Dál Riata, a number of interpretations have been offered of the relations between Óengus, Eadberht and Æthelbald in the period from 740 to 750.

This rests largely on a confused passage in Symeon of Durham's Historia Regum Anglorum, and it has more recently been suggested that the interpretation offered by Frank Stenton — that it is based on a textual error and that Óengus and Æthelbald were not associated in any sort of joint overlordship — is the correct one.

He may have been appointed by Æthelbald, King of Mercia, whose councilor he was.

This action may have been taken in response to Boniface's complaints about Cuthbert and Æthelbald to the papacy.