Each year, during the first 10 days of the month of Muharram — according to the Islamic calendar — Shiite Muslims commemorate the death of Imam Hussein bin Ali, who was the grandson of the Prophet Muhammad, the son of Imam Ali (the husband of the prophet’s daughter) and one of the 12 imams who are held in the highest esteem among Shiites.

Hussein was killed in the year 680, when the army of Yazid bin Muawiya cut off the road that Hussein was on as he was going to Iraq. According to the narrative, Hussein sought to save his grandfather’s religion from the misguided Umayyad rule.

Yazid’s army surrounded Hussein and his family in the central Iraqi city of Karbala for three days. The siege ended with the killing of Hussein and his men and the capture of the women.

Since that date, Shiites have commemorated this event with great fervor, as if it happened yesterday. In principle, there is no deep religious disagreement between Sunnis and Shiites on whether Hussein was treated unjustly and over the validity of his goals. But they do disagree about the significance of Karbala in the subsequent conflict over the legitimacy of the reigning power after the death of the Prophet Muhammad. This disagreement is so deep that it caused a clash between the two sides that still continues to this day.