The Abbasids, a branch of Islam whose Imam was descended in a line from an uncle of the Prophet Muhammad, overthrew the ruling Umayyad line of caliphs who ruled from Damascus around the year 750. Baghdad became the seat of power for the Abbasids when they took control of the Dar al-Islam ("House of Islam," or the lands ruled by the caliphate). When the Abbasids entered Damascus and the royal palace there, a terrible slaughter ensued, with all who fled being chased by horsemen and cut down as they ran.
The various divisions within Islam were, and still are, frequently as brutal towards other branches of Islam as they were to other religions. The same can be said of Christianity. It can also be said that these two religions, Christianity and Islam, more than any others, have employed the sword to spread their beliefs to those who did not previously share them. But I believe that fact is more of a testament to the time that these religions took root and the state of humanity at that time than anything that can be attributed to the compassionate belief systems that the religions intended to promote. Although I believe Christ was the Son of God and the Son of Man, who never led any battles or killed another person or sought the death of anyone at any place or any time, whereas Muhammad was a military leader who directed the slaughter of thousands in battle and enjoyed the fruits of that astonishing string of victories, I also know that many (not all) of the teachings of Islam and of Christianity are virtually identical in spirit and in practice. Muhammad was directly responsible for transmitting the words of the Qu'ran to humanity, whether the Angel Gabriel (Jibril) delivered them straight from God (Allah) or not, and for that, at that brutal period of mankind's brutal history, the world owes much thanks.
At various times in the histories of each religion, it was acceptable to proclaim one's belief and be okay with the fact that your neighbor held a different one (even though you felt bad for him and the future of his soul). It is mostly that way today. For this I am grateful, and more than that, I am hopeful.