As Iraq descends into chaos and civil war, FRONTLINE examines the rise of its neighbor -- Iran -- as one of America's greatest threats and most puzzling foreign policy challenges. Through interviews with key players on both sides, FRONTLINE traces the tumultuous history of U.S.-Iran relations since 9/11 -- from unprecedented early cooperation in Afghanistan, to the growing crisis over Iran's nuclear ambitions and Tehran's open threats to drive America out of the Middle East.