She married Æthelred, Lord of Mercia, and ruled at his side until his death in 911. During this time she played an active role in Mercia’s defense against the Vikings, forcing them into a temporary peace. This, however, was nothing on her actions after her husband’s death when she was proclaimed “Lady of the Mercians” and sole ruler of Mercia.
Over the course of the next four years Æthelflæd built and very successfully defended a series of fortifications in the north against the incursions of the Vikings. She also reconquered Leicester and Derby and in 917 led an army into Wales to avenge the murder of a Mercian abbot, subduing the local prince and returning victorious to Mercia.
Her achievements were not only military in nature, but political as well. Her alliance with her brother Edward the Elder was what allowed him to accomplish much of what he did during his reign and vice versa. She forged alliances with peoples of all sorts, from the king of Alba (Scotland) to a few of the Viking leaders. In 918, the people of York promised to pledge her their loyalty, probably in return for military support, but she died just weeks before the promise could be carried out.
Two factions sprang up among the Mercians after Æthelflæd’s death, one in support of her brother Edward the Elder and his son Æthelstan (who had been brought up in his aunt’s court and therefore had the opportunity to make political connections) and one in support of her daughter Ælfwyn on the basis of her birth, her mother’s career, and quite probably her own training. It was Ælfwyn who took control of Mercia after her mother’s death. Six months into her rule, however, her uncle Edward forced her out of power and took his niece’s lands for himself.
*If you ever have trouble remembering his name, just think Alfred the Cake. It comes from the book 1066 And All That and I’ve found it’s just ridiculous enough to stick.
Stafford, Pauline. Queens, Concubines, and Dowagers: the King’s Wife in the Early Middle Ages. Athens, Georgia: University of Georgia Press, 1983.
Asser's LIfe of Alfred (Translated by Albert Cook) - Archive.org (see section 75)
Æthelflæd - Wikipedia
Charters - Anglo-Saxons.net