The Second Fitna
The Second Fitna, or Second Islamic Civil War, was a period of general political and military disorder that afflicted the Islamic world during the early Umayyad dynasty, following the death of the first Umayyad caliph Muawiyah I. There seems to be a lack of solid consensus on the exact range of years that define the conflict, with several different historians dating the Second Fitna differently. Some see the end of Muawiya's reign in 680 AD as marking the beginning of the period, while the year 683 (following the death of Muawiya's son the Caliph Yazid I) is cited by others. Similarly, the end is variously dated from 685 (after the ascension of Caliph Abd al-Malik ibn Marwan) to 692 (following the death of Ibn al-Zubair and the termination of his revolt). The dates 683-685 seem to be the most commonly used.
The first Umayyad Caliph Muawiya I was succeeded upon his death in 680 by his son, Yazid I. Yazid's first opposition came from supporters of Husayn ibn Ali, who was the grandson of the Prophet Muhammad and the son of the former Caliph Ali ibn Abi Talib, who had been assassinated. Husayn and many of his closest supporters were killed by Yazid's troops at the Battle of Karbala. This battle is often cited as the definitive break between the Shi'a and Sunni sects of Islam, and until this day it has been commemorated each year by Shi'a Muslims on the Day of Ashura.