First Fitna: In the Battle of Siffin, troops led by Ali ibn Abu Talib clash with those led by Muawiyah I.
The Battle of Siffin was fought in 657 CE (37 AH) between Ali ibn Abi Talib, the fourth of the Rashidun Caliphs and the first Shia Imam, and Muawiyah ibn Abi Sufyan, the rebellious governor of Syria. The battle is named after its location, Siffin, on the banks of the Euphrates. The fighting stopped after the Syrians, faced with overwhelming odds of defeat, called for arbitration. The arbitration process ended inconclusively in 658 CE. The battle is considered a part of the First Fitna.
The First Fitna (Arabic: فتنة مقتل عثمان, romanized: fitnat maqtal ʻUthmān, lit. 'strife/sedition of the killing of Uthman') was the first civil war in the Islamic community which led to the overthrow of the Rashidun Caliphate and the establishment of the Umayyad Caliphate. The civil war involved three main battles between the fourth Rashidun caliph, Ali, and the rebel groups.
The roots of the first civil war can be traced back to the assassination of the second caliph, Umar. Before he died from his wounds, Umar formed a six-member council, which ultimately elected Uthman as the next caliph. During the final years of Uthman's caliphate, he was accused of nepotism and eventually killed by rebels in 656. After Uthman's assassination, Ali was elected the fourth caliph. Aisha, Talha, and Zubayr revolted against Ali to depose him. The two parties fought the Battle of the Camel in December 656, in which Ali emerged victorious. Afterwards, Mu'awiya, the incumbent governor of Syria, declared war on Ali ostensibly to avenge Uthman's death. The two parties fought the Battle of Siffin in July 657. This battle ended in stalemate and a call for arbitration, which was resented by the Kharijites, who declared Ali, Mu'awiya, and their followers as infidels. Following the Kharijites' violence against civilians, Ali's forces crushed them in the Battle of Nahrawan. Soon after, Mu'awiya also seized control of Egypt with the aid of Amr ibn al-As.
In 661, Ali was assassinated by the Kharijite Abd al-Rahman ibn Muljam. After Ali's death, his heir, Hasan, was elected caliph and soon after attacked by Mu'awiya. The embattled Hasan concluded a peace treaty, acknowledging the rule of Mu'awiya. The latter founded the Umayyad Caliphate and ruled as its first caliph.