Æthelflæd, Lady of the Mercians (c. 870 – 12 June 918), ruled Mercia in the English Midlands from 911 until her death. She was the eldest daughter of Alfred the Great, king of the Anglo-Saxon kingdom of Wessex, and his wife Ealhswith. Æthelflæd was born around 870 at the height of the Viking invasions of England. By 878 most of England was under Danish Viking rule, East Anglia and Northumbria having been conquered, and Mercia partitioned between the English and the Vikings, but in that year Alfred won a crucial victory at the Battle of Edington. Soon afterwards the English-controlled western half of Mercia came under the rule of Æthelred, Lord of the Mercians, who accepted Alfred's overlordship. Alfred adopted the title King of the English, claiming to rule all English people not living in areas under Viking control. In the mid-880s, Alfred sealed the strategic alliance between the surviving English kingdoms by marrying Æthelflæd to Æthelred.
Æthelred played a major role in fighting off renewed Viking attacks in the 890s, together with Æthelflæd's brother, the future King Edward the Elder. Æthelred and Æthelflæd fortified Worcester, gave generous donations to Mercian churches and built a new minster in Gloucester. Æthelred's health probably declined early in the next decade, after which it is likely that Æthelflæd was mainly responsible for the government of Mercia. Edward had succeeded as King of the Anglo-Saxons in 899, and in 909 he sent a West Saxon and Mercian force to raid the northern Danelaw. They returned with the remains of the royal Northumbrian saint, Oswald, which were translated to the new Gloucester minster. Æthelred died in 911 and Æthelflæd then ruled Mercia as Lady of the Mercians. The accession of a female ruler in Mercia is described by the historian Ian Walker as "one of the most unique events in early medieval history".
Alfred had built a network of fortified burhs and in the 910s Edward and Æthelflæd embarked on a programme of extending them. Among the towns where she built defences were Bridgnorth, Tamworth, Stafford, Warwick, Chirbury and Runcorn. In 917 she sent an army to capture Derby, the first of the Five Boroughs of the Danelaw to fall to the English, a victory described by Tim Clarkson as "her greatest triumph". In 918 Leicester surrendered without a fight. Shortly afterwards the Viking leaders of York offered her their loyalty, but she died on 12 June 918 before she could take advantage of the offer, and a few months later Edward completed the conquest of Mercia. Æthelflæd was succeeded by her daughter Ælfwynn, but in December Edward took personal control of Mercia and carried Ælfwynn off to Wessex.
Historians disagree whether Mercia was an independent kingdom under Æthelred and Æthelflæd but they agree that Æthelflæd was a great ruler who played an important part in the conquest of the Danelaw. She was praised by Anglo-Norman chroniclers such as William of Malmesbury, who described her as "a powerful accession to [Edward's] party, the delight of his subjects, the dread of his enemies, a woman of enlarged soul". According to Pauline Stafford, "like ... I she became a wonder to later ages". In Nick Higham's view, medieval and modern writers have been so captivated by her that Edward's reputation has suffered unfairly in comparison.