Bede in his Ecclesiastical History includes his…

Bede in his Ecclesiastical History includes his list of seven kings who held "imperium" over the other kingdoms south of the Humber.

The usual translation for "imperium" is "overlordship".

Bede names Æthelberht as the third on the list, after Ælle of Sussex and Ceawlin of Wessex.

The anonymous annalist who composed one of the Anglo-Saxon chronicles repeated Bede’s list of seven kings in a famous entry under the year 827, with one additional king, Egbert of Wessex.

The Chronicle also states that these kings held the title "bretwalda", or "Britain-ruler".

The exact meaning of bretwalda has been the subject of much debate; it has been described as a term "of encomiastic poetry", but there also is evidence that it implied a definite role of military leadership.

The prior bretwalda noted, Ceawlin, is recorded by the Anglo-Saxon Chronicle as having fought Æthelberht in 568.

The entry states that Æthelberht lost the battle and was driven back to Kent.

The dating of the entries concerning the West Saxons in this section of the Chronicle is thought to be unreliable and a recent analysis suggests that Ceawlin’s reign is more likely to have been approximately 581–588, rather than the dates of 560–592 that are given in the Chronicle.

The battle was at "Wibbandun", which may be translated as Wibba’s Mount; it is not known where this was.

At some point Ceawlin ceased to hold the title of bretwalda, perhaps, after a battle at Stoke Lyne, in Oxfordshire, which the Chronicle dates to 584, some eight years before he was deposed in 592 (again using the Chronicle’s unreliable dating).

Æthelberht certainly was a dominant ruler by 601, when Gregory the Great wrote to him, urging Æthelberht to spread Christianity among those kings and peoples subject to him, implying some level of overlordship.

If the battle of Wibbandun was fought circa 590, as has been suggested, then Æthelberht must have gained his position as overlord some time in the 590s.

This dating for Wibbandun is slightly inconsistent with the proposed dates of 581–588 for Ceawlin’s reign, but those dates are not thought to be precise, merely the most plausible given the available data.