Complex sentence: After the disaster at Velbazhd, the Byzantines attacked Bulgaria seizing a number of towns and castles in northern Thrace but their success was reverted in 1332 when the new emperor Ivan Alexander defeated them in the battle of Rusokastro and recovered the lost territories.
Split into simpler sentences: After the disaster at Velbazhd, the Byzantines attacked Bulgaria, seizing a number of towns and castles in northern Thrace. Their success ended in 1332, when the new emperor Ivan Alexander defeated them in the battle of Rusokastro, recovering the lost territories.