The Battle of Karbala took place on Muharram 10, in the year 61 of the Islamic calendar (October 9 or 10, 680) in Karbala, in present day Iraq. On one side were supporters and relatives of Muhammad's grandson Husayn ibn Ali; on the other side was a military detachment from the forces of Yazid I, the Umayyad caliph.
Husayn ibn Ali's group consisted of notable members of Muhammad's close relatives, around 128, of whom some were either old (90+ years) or young (6 months). Husayn and some members of his group were accompanied by some of the women and children from their families. On the opposing side, the armed forces of Yazid I were led by Umar ibn Sa'ad.
The reason for the battle was Yazid's insistence that Husayn accept him (ie Yazid) as the righteous and undisputed ruler. Husayn was no ordinary person. Grandson of the prophet, he was widely accepted as the most pious throughout his life. A seal from such a personality would have given a kiss of life to Yazid's unlawful power grab. Both were well aware of this. Yazid chose to use the state power to first deny access to water and food for three days to Husayn and his family (including women and children) and when Husayn still insisted on his stance of not providing legitimacy to Yazid's rule, told Husayn that he would be killed. Husayn chose that. He and his followers were killed by the forces of Yazid during the 1 day battle in Karbala. The captured women were tortured, mocked and made an example of. In the end though, these women, through their speeches were able to turn the tables on Yazid.
This battle also had significant effects on the formation of revolts against the Umayyad dynasty.
The battle of Husayn ibn Ali is commemorated during an annual 10-day period held every Muharram by the Shi'ah as well as many Sunnis, culminating on its tenth day, Ashura.