There really was no 'fundamental division in theology". Silly as itmay sound in view of IS atrocities and the Sunni / Shia fights inIraq and Syria, the major difference betwee
…n the two beliefs ispurely the result of a dispute over leadership in the 7th (!)century AD. The Sunnis think that a guy named Abu Bakr and hisfamily was the prophet Muhammad's rightful successor, while theShiites thought that is was Muhammad's son-in-law Ali. The theological differences are few and all very minor. Until theend of the 20th century, Shiites and Sunnites generally livedpeacefully together and intermarriage was common. Today's division is mostly the result of an ongoing struggle forpolitical dominance of the Middle East between Sunni Saudi Arabiaand Shiite Iran - plus in the past few years, Sunni Turkey thathankers for a return of some of its former Ottoman Empiregreatness. ( Full Answer )