Armed conflicts in Ukraine and Gaza are very visible reminders of the severe negative impacts that armed conflicts have on the environment. Yet, despite knowledge of singular environmental issues and specific country contexts, the magnitude, temporal, and multiscalar impact of armed conflict on broader environmental performance has not been quantified. This knowledge gap limits the ability to design broad and targeted measures of environmental protection during armed conflict as well as rehabilitation in the post-conflict period. Here, we conduct the first global study on the environmental impacts of armed conflict. Our analysis shows that countries with an armed conflict have a significantly worse environmental performance. Armed conflict length and severity are both negatively affecting the environmental performance of countries. Even after an armed conflict ends, countries need about 20 to 30 years to recover in terms of environmental performance. This finding is particularly concerning when considering the risk of conflict recurrence due to environmental stress and bad natural resource management. Taken together, this demonstrates the urgency of measures to protect the environment during armed conflicts and in post-conflict settings.