how and why did muslims become divided into different groups
There are two main branches of Islam: Sunnism and Shi'ism. Sunni Muslims believe that the successor of Muhammad should be elected. Sunni Muslims make up the majority of all Muslims. Shi'ites believe that the successor of Muhammad should be blood-related to Muhammad.
TL;DR Politics. Basically the same way, there are New Atheists, and Old Atheists. The Sunni and Shi'a split was really focused on who would continue to lead the people. Sunni's believed in a process where the people would elect a leader, while the Shi'a group believed that there should be blood relation. It has nothing really to do with being the successor of Muhammad, as both divisions believe he is the final prophet. Along side the differences on who would continue to lead the people, there are now differences between Sunni, and Shi'a fiqh, and hadith. I could be expansive and cover Ibn Abbas's commentary, if you would like. However that only covers one viewpoint.
@Aveline @Opcode thanks alot
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