The track to be explored in this paper was laid down when I realised how relatively unexamined the actual working out of Christian ideas about war within the medieval period is. Recent years have seen appear a notable book about the development of ideas on the Just War, and a great deal of work on the role of the military aristocracy and on its ideals, but upon the coming together of Christianity and actual events there seemed to me very little, at least in the period which interests me most. The one series of events which has attracted attention within what one can call loosely the twelfth century is, of course, the Crusades, but I decided to put them rather at the edge of my focus since they raised special questions, and to invite a scholar who has devoted much time to their elucidation to give a paper upon a crusading theme later in the conference. Yet when one turns for guidance for the history of western Europe there is only one book which stands out, La Guerre au Moyen Age by Philippe Contamine which appeared in the Nouvelle Clio series as recently as 1980, and it, as one would expect from its author’s earlier achievement, is strongest when it deals with the period of the Hundred Years War. Nonetheless it is a remarkable achievement, and one to which I am deeply indebted. But given the fact that the subject is still so unmapped, only two approaches seemed feasible to me, one where I would try to look at a series of specific wars and see what the Church did about them, or one where I would look at a source or group of sources, and see what it, or they, had to say about war and the Church.