On the tenth of Muharram in the year 61 of the Islamic era, a party of some seventy people was surrounded, cut off from the Euphrates and killed on a plain in Iraq. Husayn ibn Ali, grandson of the Prophet, had been invited by the people of Kufa to lead them against the Umayyad caliph Yazid, and had set out with his household. The Kufan support collapsed under the governor's pressure before he arrived, and an army was sent to intercept him.
The details preserved in the tradition are domestic rather than military, and this is why the event carries the weight it does. The party was denied water for days within sight of the river. The men were killed one by one, including Husayn's half-brother Abbas, who died attempting to bring water to the children, and an infant son. The women and the surviving son were taken captive to Kufa and then Damascus, where Husayn's sister Zaynab is remembered for denouncing the caliph to his face.
Militarily this was a minor action in a failed rising. Its significance is entirely religious, and it was established quickly. Within a generation there were penitential movements among those who had invited Husayn and failed to support him, and the plain became a place of pilgrimage while the Umayyads still ruled.