Complex sentence: By 878 the Vikings had destroyed the kingdoms of East Anglia, Northumbria, and Mercia, and nearly conquered Wessex, but the West Saxons fought back under Æthelstan's grandfather, Alfred the Great, and achieved a decisive victory at the Battle of Edington.
Split into simpler sentences: By 878 the Vikings had destroyed the kingdoms of East Anglia, Northumbria, and Mercia, and nearly conquered Wessex. The West Saxons fought back under Æthelstan's grandfather, Alfred the Great, and achieved a decisive victory at the Battle of Edington.