Although Æthelred is not recorded until 883, historians believe that he was “Edryd Long-Hair”, the leader of a Mercian army which invaded Gwynedd in 881, and was defeated by Rhodir Mawr’s sons at the battle of the Conwy. This was described by Welsh annals as “revenge by God for Rhodri”. The battle of Conwy forced Æthelred to abandon his ambitions in north Wales, but he continued to seek overlordship over the south-eastern Welsh kingdoms of Glywysing and Gwent. According to Alfred’s Welsh biographer Asser, Æthelred’s “might and tyrannical behaviour” forced these kingdoms to seek the protection of King Alfred‘s lordship. Alfred was happy to accept, and the defeat at Conwy may have forced Æthelred to accept Mercian submission to Wessex in 883, an important step in creating a single English kingdom.
In 886, Æthelred was given control of London by Alfred. As Alfred had conquered the southern Danelaw, it was useful to place the ruler of the divided Kingdom of Mercia in control of its former eastern region. In 892 the Vikings launched the last invasion of Wessex in Alfred’s reign, and the following year the main army marched from Essex through Mercia to the Welsh border, where it was defeated by a joint Mercian and Welsh army under Æthelred at the Battle of Buttington, in the most decisive victory of the war. The Anglo-Saxon Chronicle says that Æthelred spent much of the 890s leading military campaigns in eastern England.