Sunnis Vs Shi'a - Divided on Iran?

The differences between the Sunni and Shiite religious beliefs offers a lot toward explaining the ongoing conflict in Iraq, as well as century long conflicts between nations in the Middle East.
The Sunni branch believes that the first four caliphs--Mohammed's successors--rightfully took his place as the leaders of Muslims. They recognize the heirs of the four caliphs as legitimate religious leaders. These heirs ruled continuously in the Arab world until the break-up of the Ottoman Empire following the end of the First World War. Shiites, in contrast, believe that only the heirs of the fourth caliph, Ali, are the legitimate successors of Mohammed. But which way Iran?