The Anglo-Saxon Chronicle is the main source of information for the period of Ethelfleda. For a short time a separate Chronicle was produced in Mercia, recounting the successes of the Lady of the Mercians. The reference to the foundation of Runcorn, which is not included in my translation of the Anglo-Saxon Chronicle, must have come from the Mercian Chronicle.
I read that Ethelfleda was given equal status to Ethelred when she married him. This seems unlikely as she was only 15 years old, but if she did not have equal power at that stage, it is clear that she acquired power during their marriage as she signed important documents during his lifetime and took formal control of Mercia on his death. Ethelfleda ruled for approximately eight years (according to the Anglo-Saxon Chronicle) from the newly fortified Mercian capital at Stafford,
Her brother Edward, who had succeeded Alfred, depended heavily on his Mercian allies and trusted her to act independently in their war against the Danes. This allowed him to carry out separate complimentary campaigns against the Danes. The Anglo-Saxon Chronicle informs us that when she died she was 'with rightful lordship holding Mercian rule'. However, this power was clearly due to her personal qualities, not just her position as Lady of Mercia. We know this because when she died, Edward did not want her daughter Aelfwynn to act independently and a few months after the death of Ethelfleda, he took direct control of Mercia himself as part of England. Aelfwynn was sent to a monastery.
Elthefleda commanded armies in the field, but it is unlikely that she fought in hand to hand combat. But we can imagine her standing behind the formidable shield of Anglo-Saxon warriors, giving orders, inspiring her troops and winning the respect of her enemies. She led the Mercian armies in person on expeditions she herself had planned. This has been the role of most commanders of large armies throughout history.
She built ten burhs at the rate of two a year and their sites can be traced along the Welsh border, in central England and along the Mersey Valley., The record of fortresses she built for the protection of Mercia shows she had an eye for country and the ability to forecast the movements of her enemies.
Elthefleda was the first Anglo-Saxon warrior leader. However women exercised more power that we might imagine in Anglo-Saxon England. The earlier Abbess Hilda of Whitby had been in charge of a so-called double-house of monks and nuns who answered to the abbess.