Complex sentence: A small number of extremely wealthy, landowning aristocrats occupied most seats in the Senate, and competed among themselves for election to the state's most prestigious civil offices, and monopolised the highest military and religious offices.
Split into simpler sentences: A small number of extremely wealthy, landowning aristocrats occupied most seats in the Senate. They competed among themselves for election to the state's most prestigious civil offices, and held virtual monopoly of the highest military and religious offices.