The medieval development of theories of war

In the preface to his Tree of Battles, written in 1387 and dedicated to Charles VI of France, Honoré Bouvet laments that ‘all holy Christendom is so burdened by wars and hatreds, robberies and dissensions, that it is hard to name but one little region, be it duchy or county, that enjoys good peace’. War was the normal condition of society in medieval Europe; and pessimistic doctors argued, on theological or astrological grounds, that ‘in this age it is necessary for there to be wars, and the slaughters and infinite sufferings of war’. Some men were dazzled by the pomp and circumstance of glorious war; a most doubtless agreed that ‘warres & bataylles shold be acursed thyng, & not due’.

About that cursed thing arose a prodigious literature – legal and theological, philosophical and practical, historical, strategical, and ecclesiastical. The centrepiece of the medieval discussions, to which they owe their abiding philosophical interest, is the theory of just war.

That theory is now most familiar from Aquinas' brief essay De hello (ST, Ilallae, q. 40); but in this instance Aquinas was no innovator: he stands in a long line of theorists, the fons et origo of whose ruminations is to be found in the writings of Augustine. The scattered observations of Augustine and his successors were collected and ordered by the canon lawyers of the twelfth century, whose work is best represented by Gratian's Decretum.