Abstract
The decision by the Council of Europe to ter minate the Russian Federation’s membership of the Council of Europe on 16 March 2022 makes the issue of legal certainty for aliens actively participating in the war in Ukraine as part of the Ukrainian Armed Forces completely unpredictable. The academic literature and the case law of the European Court of Human Rights in the field of the legal status of alien combatants is limited, and the International Criminal Court has not complet ed any cases on this topic. This article addresses the prin ciple of case law and, above all, the principle of legality with regard to aliens and their active participation in the armed forces of Ukraine. This issue has become cen tral since the Russian Federation may or may not grant these persons the status of prisoner of war according to the Third Geneva Convention, relating to Protocol I, or may characterise them as criminal offenders or terro rists. Keywords: aliens, combatants, mercenaries, prisoners of war, war, armed conflict, terrorists