This article examines the extent to which international legal obligations aimed at protecting the environment apply to military activities in peacetime and during armed conflict. The discussion draws on international environmental law, human rights law, the law of armed conflict, and the law of State responsibility in evaluating the extent to which States have a duty to prevent or mitigate environmental harm and remediate or compensate for any such damage caused by their military activities. The article also examines international law on liability for the injurious consequences of lawful activities, to assess whether this equitable doctrine supports shifting the clean-up costs of environmental harm to the acting State even when there is no breach of international law. The article concludes that international law requires measures be taken to prevent environmental harm and could support a claim for remediation or compensation where norms of international law have been breached. It also suggests the need to develop specific rules in peace treaties and status of forces or bases agreements, to address the consequences of environmental harm resulting from military activities.