Mortally wounded in battle
In the same year as Æthelred's succession as king, a great Viking army arrived in England, and within five years they had destroyed two of the principal English kingdoms, Northumbria and East Anglia. In 868 Æthelred's brother-in-law, Burgred king of Mercia, appealed to him for help against the Vikings. Æthelred and his brother, the future Alfred the Great, led a West Saxon army to Nottingham, but there was no decisive battle, and Burgred bought off the Vikings. In 874 the Vikings defeated Burgred and drove him into exile.
In 870 the Vikings turned their attention to Wessex, and on 4 January 871 at the Battle of Reading, Æthelred suffered a heavy defeat. Although he was able to re-form his army in time to win a victory at the Battle of Ashdown, he suffered further defeats on 22 January at Basing, and 22 March at Meretun.
In about 867, Æthelred effectively established a common currency between Wessex and Mercia by adopting the Mercian type of lunette penny, and coins minted exclusively at London and Canterbury then circulated in the two kingdoms.
Æthelred died shortly after Easter (15 April) 871, and is buried at Wimborne Minster in Dorset. He was succeeded by his younger brother, Alfred the Great (wikipedia.org)