Brief History

I am reading "The Middle East" A brief history of the last 2,000 years by Bernard Lewis. Of particular interest is the section on the period of the Crusades. A discussion of a period about 1184 when Jerusalem was a city where Musims and Christians lived together is of interest to me since it is coverd by the movie by Ridely Scott called the "Kingdom of Heaven" Even during this period the Sunnis and Shites were fighting between themselves. It seems from what I have read so far in the book the Muslims have a long history of internal fighting within the Islam religion.