The great divide between the Sunni and Shia isn't the only religious division in Islam but it's the biggest the first and most important one . Today 80% of Muslims worldwide are Sunni while 15% are Shia but unlike Sunni Shia are more unified and have hierarchy similar to Catholicism
The divide began 25 years after prophet Muhammad's death and it was called the first Fitnah or the biggest Fitnah .
After Muhammad's death in 632 Muslims chose Abu Bakr as a caliph (the word Khalifa means successor in Arabic ) . The caliph is considered the successor of the prophet politically and religiously so he is both the leader of the state and the mosque .
In 644 the second Caliph Omar died and Usman became the third Caliph .
Usman put his family relatives in most crucial government positions and made them also the governors of provinces including Muayiah the governor of Syria and unlike Omar he gave regional leader more authority this angered a lot of people throughout the caliphate and after years of preparations and meetings so many people from all regions (Yemen . Oman . Hijaz . Arabia . Iraq . Persia . Egypt . Palestinian . Syria ) gathered in Medina year 656 and they surrounded Usman's house for 80 days of failed negotiations .
During the negotiations Usman refused to back down since he is the caliph thus he represented the prophet on the world and can't be replaced by ordinary humans . There was also the important factor that there was technically no limit to the caliph authority and no legitimate mechanisms to replace a caliph and after 80 days of getting no result a group of rebels were so frustrated and angry that they made the biggest disaster in human history they attacked the caliph in his house and killed him in 656 .
This put the entire Islamic world in chaos till today . The chaos resulted from the assassination started the big Fitnah and got tens of millions of people dead up to today .
It's estimated that 150 millions are killed between 657 and 2019 .
The first map is the map of Islam today
The secondone is the caliphate state during Usman's rule