In looking back, with our present experience, we are driven to the melancholy conclusion that, instead of diminishing the number of wars, ecclesiastical influence has actually and very seriously increased it. We may look in vain for any period since Constantine, in which the clergy, as a body, exerted themselves to repress the military spirit, or to prevent or abridge a particular war, with an energy at all comparable to that which they displayed in stimulating the fanaticism of the crusaders, in producing the atrocious massacre of the Albigenses, in embittering the religious contests that followed the Reformation.

Chapter 5 (3rd edition p. 254) - A History of European Morals from Augustus to Charlemagne (1869)