Environmental destruction in war -- does that fall under the purview of ethics? Some claim that war itself is not a subject of ethics; others hold that ethics deals strictly with interhuman relationships, not with the relationship between human beings and nature. We argue that the just war (bellum justum) tradition of moral reflection points in another direction. It provides an ethical vocabulary for assessing the impact of war on our natural environment. After some introductory comments on the relationship between ethical inquiry and the international laws of armed conflict, this article considers the conceptions of war and of nature which underlie just war thinking, with special emphasis on St Thomas Aquinas's formulation of this tradition. It specifically addresses two well-known debates: the debate between pacifists and realists on the one hand, and the debate between anthropocentric and non-anthropocentric views on nature on the other. Then, through an exposition and application of the criteria of competent authority, just cause, right intention, discrimination, and proportionality, as well as the rule of double effect, the article shows how just war arguments can clarify contemporary moral thinking about environmental damage in war.