Complex sentence: With a lack of well respected political leaders division in Athenian society increased, and by 411 BCE the clubs known as "hetairiai" gained increasing political importance and were an outlet for those who opposed the system of democracy.
Split into simpler sentences: With a lack of well respected political leaders, division in Athenian society increased. By 411 BCE the clubs known as "hetairiai" gained increasing political importance and were an outlet for those who opposed the system of democracy.