When the prophet Mohammad died, he had not named a successor. So, his Muslim followers were ripped apart by which successor to support: 1) Mohammed’s brother-in-law or 2) his uncle. Neither side has conceded and so there's been 1,400 years of war between the two factions. There have been other sects of Islam, but they have all split along the Sunni vs. Shi'ite division.
The war continues in Syria. Assad's tribe is the Alawite Muslims that are supported by Shi'ite Muslims, Druze Muslims, and Christians. They are 26% of the population that run Syria. The rebels are predominately Sunni Muslims, as are 74% of the country. They are backed by the Saudis (Sunni) and Assad is backed by Iran (Shi'ite). That's the story in a nut shell. There are no good guys here; there are bad guys and worse guys. They are all murders and thugs.
The Christian world was split between Catholics and Protestants. Neither side was tolerant of the other and this led to the sectarian fighting between the supporters of the Pope and followers of the heretics John Wycliffe, Jan Hus, Martin Luther, John Calvin, and others. The wars and killing of the Great Schism devastated Europe [1517 to 1648] but didn't result in the supremacy of one branch of Christianity over the other. Eventually both sides began to tolerate each other, albeit reluctantly.
The problem for Islam is no such Reformation has occurred; they are still fighting their equivalent of the wars of the Reformation that's gone on for 1,400 years without resolution.
A thousand thanks.