Complex sentence: Rome's majority of citizen commoners (plebs) were ruled by a small number of wealthy and powerful patricians (aristocrats) who asserted a traditional, exclusive right to Rome's highest religious, political and military offices.
Split into simpler sentences: Rome's majority were citizen commoners (plebs), ruled by a small number of wealthy and powerful patricians (aristocrats). The patricians asserted their traditional, exclusive right to Rome's highest religious, political and military offices.