<P> Shia sources further claim that by the orders of the tenth Abassid caliph, al - Mutawakkil, the tomb of the third Imam, Hussein ibn Ali in Karbala, was completely demolished, and Shias were sometimes beheaded in groups, buried alive, or even placed alive within the walls of government buildings still under construction . </P> <P> The Shia believe that their community continued to live for the most part in hiding and followed their religious life secretly without external manifestations . </P> <P> Many Shia Iranians migrated to what is now Iraq in the 16th century . "It is said that when modern Iraq was formed, some of the population of Karbala was Iranian". In time, these immigrants adopted the Arabic language and Arab identity, but their origin has been used to "unfairly cast them as lackeys of Iran". However many of these Shias come from Sayyid families with origins in tribes from Iraq, Lebanon and Bahrain, one of said tribes being al - Musawi, and two prevalent families that are descended from it and lived in Iran for some time before settling in Iraq are the al - Qazwini and al - Shahristani families . Other Iraqi Shias are ethnic Arabs with roots in Iraq as deep as those of their Sunni counterparts . </P> <P> Shafi'i Sunnism was the dominant form of Islam in most of Iran until rise of the Safavid Empire although a significant undercurrent of Ismailism and a very large minority of Twelvers were present all over Persia . </P>

Explain the split between the shiites and the sunnis