Abbas Milani in The New Republic:
What we are witnessing right now in the streets of Tehran is, first and foremost, a political battle for the future of the Iranian state. But closely linked to this political fight is also an old theological dispute about the nature of Shiism–a dispute that has been roiling Iran for more than a century.
Shiism, like most religions, is no stranger to heated schisms. Shia and Sunnis split over the question of whether Muhammad had designated his son-in-law, Ali, as his successor (Shia believed he had). Some Shia, called Alawites, believe the only divinely designated successor was Ali, while another group, Zaydis, believe there were four imams. A large, intellectually vibrant third group is known as the Ismailis because it believes the line of imams ended with the seventh, Ismail. And the largest Shia sect is called the Ithna Ashari–or the Twelvers. Dominant in Iran, they believe in twelve imams and posit that the last imam went into hiding some 1,100 years ago. His return, bloody and vengeful, will mark the redemptive dawn of the age of justice.
It is within this branch that a further split took place beginning in the late nineteenth century–the moment when the Iranian elite began to confront the challenge of modernity. Ideas like rationalism, individualism, constitutionalism, rule of law, equality, democracy, secularism, privacy, and separation of powers began to find currency in Iran's political discourse. By 1905, these ideas, prevalent primarily among the intelligentsia, led to the Constitutional Revolution–the first of its kind in the Muslim world. The Shia clergy were faced with a historic challenge not unlike what the Catholic Church experienced with the advent of the Renaissance. How two rival ayatollahs reacted to that challenge would divide Iranian Shiism–and lay the groundwork for what is taking place today.