Throughout the 7th and 8th centuries, power fluctuated between the larger kingdoms. Due to succession crises, Northumbrian hegemony was not constant, and Mercia remained a very powerful kingdom, especially under Penda. Two defeats ended Northumbrian dominance: the Battle of the Trent in 679 against Mercia, and Nechtanesmere in 685 against the Picts.
The so-called "Mercian Supremacy" dominated the 8th century, though it was not constant. Aethelbald and Offa, the two most powerful kings, achieved high status; indeed, Offa was considered the overlord of south Britain by Charlemagne. His power is illustrated by the fact that he summoned the resources to build Offa's Dyke. However, a rising Wessex, and challenges from smaller kingdoms, kept Mercian power in check, and by the early 9th century the "Mercian Supremacy" was over.
This period has been described as the Heptarchy, though this term has now fallen out of academic use. The term arose because the seven kingdoms of Northumbria, Mercia, Kent, East Anglia, Essex, Sussex and Wessex were the main polities of south Britain. Other small kingdoms were also politically important across this period: Hwicce, Magonsaete, Lindsey and Middle Anglia.