Karbala's shrines have been destroyed by fellow Muslims more than once, and the walking pilgrimage now held there each year is among the largest gatherings of human beings anywhere on earth.
Torn down by a caliph
In 850 the Abbasid caliph al-Mutawakkil, hostile to the growing pilgrimage tradition at Husayn's grave, ordered the shrine demolished and the site ploughed and flooded to discourage visitors, a deliberate act of religious suppression rather than incidental war damage. The shrine was later rebuilt once political conditions changed, only to be sacked again in 1802 by Wahhabi forces from the Arabian interior, who killed a large number of residents and looted the shrine's treasury.
Arbaeen
Forty days after Ashura, pilgrims mark Arbaeen, commemorating the end of the traditional mourning period for Husayn, with a mass walking pilgrimage along the road from Najaf to Karbala. In recent years the gathering has drawn well over ten million participants, organised substantially through free roadside hospitality provided by volunteers along the route, making it one of the largest annual peaceful gatherings recorded anywhere, larger by attendance than the Hajj itself.
Security remains a live concern
The shrines have faced repeated threats from sectarian violence in recent decades, including during the period of Islamic State's territorial control in parts of Iraq, and remain under heavy security to this day, a precaution the site's history of deliberate destruction makes far from theoretical.