Complex sentence: From the middle of the 10th century Bulgaria was in decline and entering a social and spiritual turmoil in part due to Simeon's devastating wars and exasperated by a series of successful Byzantine military campaigns.
Split into simpler sentences: From the middle of the 10th century Bulgaria was in decline as it entered a social and spiritual turmoil. It was in part due to Simeon's devastating wars, but was also exasperated by a series of successful Byzantine military campaigns.