In “just war” thinking, proportionality is a criterion both of going to war and of fighting in it. This article uses the Battle of the Somme—a byword for immorally profligate warfare—to consider how proportionality should be understood. It reaches six conclusions: (1) a very large number of casualties is not in itself disproportionate; (2) the proportionality of a particular military operation depends on the moral standing of the larger belligerency to which it belongs; (3) aptness in the sense of being a fit response to injustice requires an account of what kinds of injustice warrant the costs of war; (4) strategy or tactics that are inefficient in the spending of lives are disproportionate; (5) if a strategist or tactician could have known that his plans were inefficient, and if we judge that he should have known, then the disproportion is culpable; nevertheless, (6) attrition can be the most efficient way of fighting.

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