Threats to environmental security ('eco-threats') face not only shortcomings of environmental protection during hostilities under the Law of Armed Conflict. Considering the new 'Hybrid Threat' concept, which had recently been discussed by NATO, the authors recognise from the perspective of International Law the need for adopting a comprehensive legal approach towards such threats. The International Environmental Law, the International Humanitarian Law and the Customary International Law show some shortcomings to tackle the new challenge. In the focus is the rule of law, in particular the principle of proportionality, which can play a role when countering such threats and legal rules can have a positive effect on the international community's ability to act.