I think we're on different pages here. To clarify, here is a timeline:

632: Schism happens. Conflict begins here.

633 - 1515: Period of occassional strife, shia uprisings (usually by pretenders for political gain), and sunni inquisitions.

1516 - 1918: Around this time the Ottoman Empire has begun conquering many parts of the Middle East, and the emperor claims to be the new Caliph of Islam. Uses this influence and central governing power to quell uprisings and mend relations. Overall, the most volatile regions involving sunni/shia civil strife are kept under watchful ottoman eyes to keep stability.

1919 - 1980: After the Ottomans were dissolved, France and Great Britain partitioned its middle eastern territories amongst themselves, and eventually granted independence. There was actually a strong movement during this period wherein several attempts were made to unify the islamic faiths, and might have succeeded if history turned out a little differently.

1981-present: Things broke down due to a lot of reasons (soured relations between Iran and Saudi Arabia, Iraq demographically divided because britain sucks at defining ethnic borders, etc.), and now it has become worse than ever due to foreign intervention causing further divide and leading to the formation of groups like ISIS.

OP said nothing about Christianity but when someone attacks the character of a minority group or at least a minority where I live it can be illuminating to point out that the majority group has done the same things. That it because these groups are made up of people, and people are all more or less the same
And yes, Islam was spread very effectively through violence but that is the history of almost any group that becomes successful. It's not that Muslims or Islam are more violent than Christians or Christianity; it's just that they're the dominant group in a region where violence is a quickest route to power