SLH
I think you have misread the early history of Christianity. Christians endured nearly three hundred years of persecution from the Roman state. It wasn't until Constantine's day that Christianity was accepted as a state religion. By then, the church was compromised and willing to use the political powers for its own ends. One has to make the distinction between the early followers of Christ and the later followers of a compromised church.
In contrast, Islam was violent from the very beginning. Mohammed used war and conquest to subject opposers. His followers used the same methods to spread the religion throughout the world.
However it should be noted that during the "Dark Ages" when Christendom was buried in superstition and ignorance, a floushing Muslim culture existed in Africa and the Middle East.