The "
Mercian Supremacy" the period of
Anglo-Saxon history between 600 and 900, when the kingdom of
Mercia dominated the Anglo-Saxon
Heptarchy.
Sir Frank Stenton apparently coined the phrase; he argued that
Offa (ruled 757–796) achieved the unification of
England south of the
Humber estuary under Mercia. Scholastic opinion on the relationship between the kingdoms of
Wessex and Mercia at this time remains divided.