Complex sentence: Caesar, who represented the populares faction in the Roman Senate, fought and won a civil war aganst the Boni, or conservative faction in the Roman Senate which left him undisputed master of the Roman world, and began extensive reforms of Roman society and government.
Split into simpler sentences: Caesar, who represented the populares faction in the Roman Senate, fought a civil war aganst the Boni, or conservative faction in the Roman Senate. When winning left him undisputed master of the Roman world, he began extensive reforms of Roman society and government.