What was the major cause of the first civil war in early Islamic society?

The first Islamic civil war or "Fitna " happened in the 7th century, after the Caliph Uthman Ibn Affan's assassination (d. 656). According to Islamic tradition, close relatives were the ones to take responsibility for avenging the assassinated's death so both his favorite wife, Aisha and his brother Ali got into a dispute over who would avenge Uthman Ibn Affan.

Ultimately the war ended by the signing a treaty in 661.