It was only when Christianity became the power that it had the capability to be a problem.
There was a time that it was a tenet of Christianity that it was "as impossible for a Christian to be a Caesar as it is for a Caesar to be a Christian."
As long as that was true, Christian warfare remained only spiritual. When it was forgotten under Constantine the change was immediate and dramatic. The ancient histories written before and after the era of the Arian controversy, Nicea, and Constantine and the histories written just 50 years afterward (Sozomen and Socrates) are dramatically different.
I remember a discussion once about the Anabaptists, who were the non-violent, radical section of the Reformation. Someone pointed out that even Anabaptists, when they had governmental power, were terribly violent (Munster, Germany), and I remember thinking that the early Christian and early Anabaptist conviction that Christians have no business running governments or fighting wars was probably a very good one.