Complex sentence: As early as 871, he and Halfdan Ragnarsson dispatched a few Viking parties to attack the Kingdom of Wessex, which remained vulnerable to Viking raids; they captured Reading, Berkshire, and set up camp within the town.
Split into simpler sentences: As early as 871, Bagsecg and Halfdan Ragnarsson dispatched a few forces to attack the Kingdom of Wessex, which remained vulnerable to raids. They captured Reading, Berkshire, and set up camp within the town.