Most Shi'ites today belong to the Twelver sect, which takes its name from the number of imams they recognize. The al-Askari Shrine important to the spirituality of the last three. Ali al-Hadi, the tenth imam, was seen as a threat by the ruling Abbasid dynasty, and forced to leave his home in Medina and live in Samarra, then the capital, where he was under constant guard by the caliphs' Turkish soldiers. He died there, as did his son Hassan al-Askari, the eleventh imam. (Shi'ites claim both were poisoned.)
At the time of his death, Hassan had a five-year-old son, Muhammad. Shortly after his father's funeral, this son vanished. In Twelver theology, this is the Occultation, and Muhammad is the Hidden Imam, who continues to guide and protect believers and will one day return as the Mahdi to inaugurate an era of peace and justice. The al-Askari shrine is the burial place of Ali al-Hadi and Hassan al-Askari, and next to the cave where the Occultation took place. Because this is the period between Ashura and Arba'in, this is roughly analogous to someone destroying the site of the Crucifixion during Lent.
(Crossposted to American Footprints.)