Nicholas Brooks noted that "the Mercians stand out as by far the most successful of the various early Anglo-Saxon peoples until the later ninth century", and some historians, such as Sir Frank Stenton, believe the unification of England south of the Humber estuary was achieved during the reign of Offa.
Nicholas Brooks märkis, et "mertsialased paistsid välja kui vaieldamatult kõige edukam erinevatest varajastest anglosaksi rahvastest kuni 9. sajandi lõpuni", ja mõned ajaloolased, nagu Frank Stenton, usuvad, et Inglismaa ühendamine lõuna pool Humberi estuaari saavutati Offa valitsusajal.