What caused the first Muslim civil war, or fitnah?

The first Muslim civil war, or fitnah (656–661 CE), was primarily triggered by the assassination of the third caliph, ‘Uthmān, in 656 CE.12

The conflict arose from several interconnected factors:

• Political Tensions: ‘Uthmān had faced growing opposition for favoring his own Meccan family, the Umayyads, in official appointments. His murder by Egyptian soldiers created a power vacuum and an urgent demand for justice.12
• Contested Succession: When ‘Alī was proclaimed the fourth caliph, he faced immediate challenges from prominent Meccan figures, including ‘Ā’ishah, Talhah, and Zubayr, who demanded vengeance for ‘Uthmān’s death.12
• Regional Rivalries: The struggle evolved into a three-way contest involving ‘Alī’s supporters in Iraq, the Meccan coalition, and the forces of Mu‘āwiyah, the governor of Syria and a kinsman of ‘Uthmān, who questioned the validity of ‘Alī’s caliphate.12

The war resulted in a lasting religious schism between Sunnis and Shī‘ah, as well as the emergence of the Khawārij, who rejected human arbitration in favor of divine judgment.12