Despite the temptation to play on the opening words of Vergil's Aeneid (‘Arma virumque cano’, ‘Of arms and a man I sing’) in the title of this book (Arma clerumque cano), reason prevailed in the end. While this is a book intended primarily for scholars, I also have in mind more general readers whose interests cover a wide spectrum – church history, war and the military, social history, law, European and U.S. history, the Middle Ages and the modern period, the crusading mentality, theories of the just war and the holy war, and the profound connections between religion and violence in the history of the West. I have therefore translated all non-English sources quoted in the text and relegated the original to the notes, except where philological problems require discussion of the meaning of particular words. I have also cited available translations of source material whenever possible. Furthermore, because so much material has never been translated and because the words of the original texts can often convince and impress a reader far more effectively than the words of the most skilled historian, I have quoted from the sources with greater frequency and at greater length than is perhaps common. Even after working on this subject for many years, I am still sometimes startled by the sources I am reading and the views they express. I have therefore chosen to let the sources speak for themselves as often as possible.