Another compelling development in the 880s was the rise of Alfred's relatives in Mercia. Alfred's brother-in-law, Æthelwulf and his kinsmen, Æthelfrith, both became Ealdormen in Mercia in the 880s. Thus, Mercia was increasingly dominated by 'Alfred's men'. However, thinking of Æthelred as an Alfredian puppet would be inaccurate. While accepting Alfred's elevated status, Æthelred also stood up for Mercian sensibilities, negotiating for Mercia to receive London (a traditionally Mercian town) and to be recognised as a semi-independent ruler of Mercia, rather than merely being another of Alfred's several Ealdormen. This was Æthelred making the most out of the declining Mercia he had inherited while overseeing its transition into what he believed was the best way forward for Mercia.
Æthelred’s belief in the Alfredian project for unity was demonstrated in the 890s, as Wessex faced a Viking invasion, perhaps larger than any it had previously faced. In 892, the Viking leader Hastein landed in Kent with 80 ships, while another allied Viking crew of 250 ships landed nearby in support of him. The larger army was swiftly defeated by Alfred’s son, Edward, at the Battle of Farnham, after which Æthelred led a Mercian force in aid of Edward as they chased the weakened invasion force out of Wessex. Hastein then led a second invasion, this time into Mercia. Alfred was defending Exeter at the time; in his absence, Æthelred led not only his Mercian army but also West Saxon armies from Somerset and Wiltshire. He met Hastein at the Battle of Butingdon, and the Vikings were defeated again. Æthelred had thus proven his loyalty to the alliance during its first real test and his ability as an organiser and commander of military forces. However, this fruitful relationship between Æthelred and his father-in-law was not to last, as Alfred died in 899, leaving a sword to Æthelred in his will, perhaps in recognition of his military prowess. Yet while some may have viewed Alfred’s death as the opportunity for Mercia to break away and become independent from Wessex, Æthelred remained committed to collaboration and unity, as he appeared at the court of Alfred’s successor, Edward, in 903 and 904, in both circumstances recognising Edward’s status as equal to the status Alfred held. Around this time, Edward also gave his eldest son, Æthelstan, to Æthelred, entrusting him to act as his guardian and foster father; thus, dynastically Æthelred remained closely tied to Wessex.
By the standards of the time, Æthelred was now an old man, probably in his late 50s by 900, and was recorded as often being ill and thus unable to always perform his leadership duties. However, in the immediate years after Alfred’s death, Æthelflæd began to play a more prominent role in Mercian politics, hosting councils alongside her husband, negotiating with invaders and building fortresses. She appears to have become Æthelred’s co-ruler, rather than just his consort, as he aged and became increasingly ill.
The Anglo-Saxons and Vikings were again at war in 909 when both began raiding one another's territory, culminating in the Battle of Tettenhall (911), where a Viking army was ambushed and routed by a combined Mercian and West Saxon army. This marked a critical turning point for the Anglo-Saxons, in which they transitioned from defending their territory to expanding it. Æthelred, however, would not live to see this period of expansion, as he died in 911, perhaps, from battle wounds at Tettenhall, although sources are divided as to whether he or Æthelstan led the Mercians at Tettenhall. Æthelred was buried at St Oswald's Priory in Gloucester. St Oswald was an early Christian Anglo-Saxon king who died in the battle to convert Mercia from paganism to Christianity. Æthelred had also previously sent an expedition into Viking territory to recover St Oswald's remains, returning them to Gloucester. In choosing St Oswald's Priory as his burial place, he could be portrayed as part of a long lineage of Anglo-Saxon rulers that fought for Christianity against paganism, beginning with Oswald.
Æthelred was a transformational figure. He ruled Mercia for over 30 years, overseeing its transformation from a divided, weakened and insecure kingdom into a vital part of an aggressive, stable and expansionary kingdom which was built upon Mercian and West Saxon unity. His wife, Æthelflæd, succeeded him, becoming Lady of Mercia in 911. With her ties to both Mercia and Wessex, she was ideally suited to this role. She quickly proved herself a strong and energetic ruler as she worked alongside her brother, Edward, to re-conquer much of the land the Mercians had previously lost to the Vikings. Æthelstan would eventually inherit both Mercia and Wessex in 924, after the deaths of Æthelflæd and Edward. The polity that Alfred, Æthelred, Edward and Æthelflæd had worked to construct would serve as a springboard for him to launch his northern invasions and become England’s first king in 927.
A year into his reign, Æthelstan granted privileges to St Oswald's Priory, Æthelred's burial site, in accordance with a 'pact of paternal piety' he had sworn to Æthelred. Thus, there is a sense that, in his foster son, Æthelstan, who was an adopted Mercian, Æthelred's legacy continued.
While Æthelred is not the hero in the story of English unification, he remains a key figure in the unification process, as he collaborated with Alfred and Edward, married and ruled alongside Æthelflæd and acted as a father figure to Æthelstan.
