Millions of Iranians unite in mourning under the red flag of martyrdom as they commemorate the seventh-century battle of Karbala. The festival of Ashura is especially poignant this year following Israel's surprise attacks on Iran, with the country's revolutionary leaders drawing strength from the power of religion. Iran's supreme leader, Ayatollah Ali Khamenei, makes a public appearance at Ashura prayers, receiving ecstatic cheers from his followers.