War is bad for the environment, yet the environmental ramifications of warfare have not been widely addressed by just war theorists and revisionist philosophers of war. The law and legal scholars have paid more attention to protecting nature during armed conflict. But because the law focuses invariably on rules mitigating the conduct of hostilities rather than on objective justice of cause, environmental jus ad bellum has been explored even less extensively than environmental ethics in war. Setting out with the presumption (...) against the use of force and its exceptions, this article considers whether environmental harm can trigger a new justification for war, at whose behest, and what might be a proportionate response to aggressive or negligent harm to nature. Force is clearly justified against military attacks. Where environmental harm is not caused by military aggression, proportionality points towards a response short of war. Full‐scale warfare will likely be counterproductive in protecting nature. This is less true if war is fought by drones destroying specific targets, or by cyber‐war, or by alternatives (or supplements) to war such as boycotts, ‘lawfare’, and ‘information/media warfare’. Responding in ways that minimize harm to nature also helps demonstrate ‘right intention’. (shrink)