ROB SCHMITZ, HOST:More than 20 million Shia pilgrims converged on the holy city of Karbala in Iraq this week. The massive annual pilgrimage commemorates the killing of Imam Husayn 14 centuries ago. So inside the Imam Husayn shrine, where the walls are covered in tiny mirrored mosaics and there are chandeliers hanging, Iranians and other pilgrims gathered around the spot where Imam Husayn was killed in the year 680. It's the battle of Karbala, when Imam Husayn, who opposed what he believed was an unfit Islamic ruler, was surrounded by a large army, deprived of water and then beheaded. There were red flags fluttering from the shrine's golden domes, the red for vengeance, for both Imam Husayn and Khamenei.