Complex sentence: In 865, the Viking Great Heathen Army launched a large scale invasion of the small kingdom of Kingdom of East Anglia and soon overran East Anglia, Mercia and Northumbria and came close to controlling most of Anglo-Saxon England.
Split into simpler sentences: In 865, the Viking Great Heathen Army launched a large scale invasion of the small kingdom of East Anglia. They overran East Anglia, Mercia and Northumbria and came close to controlling most of Anglo-Saxon England.