Ever since he became Iran's Supreme Leader, Ayatollah Ali Khamenei has had to contend with the nagging doubt that he is simply not up to the job. Unlike Cyrus the Great, who was born into one of the great Persian dynasties and acceded effortlessly to the throne on his father's death, Khamenei's rise owed everything to the patronage of Ayatollah Ruhollah Khomeini, his mentor and the founder of Iran's Islamic revolution. As Khomeini neared the end of his life, the succession became increasingly important. It was generally accepted by Iran's clerical establishment in Qom that this privilege would fall to...