“This was the heyday of [Ayatollah] Khomeini’s theocratic vision, when Iran thought it really could export its revolution across the Middle East, providing money and arms to anyone who claimed he could upend the old order.” He may yet turn out to be the avatar of Iranian democracy, but three decades ago Mir-Hossein Mousavi was waging a terrorist war on the United States that included bloody attacks on the U.S. embassy and Marine Corps barracks in Beirut. Mousavi, prime minister for most of the 1980s, personally selected his point man for the Beirut terror campaign, Ali Akbar Mohtashemi-pur, and dispatched...