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.