As long as distinct legal regimes apply to international and non-international armed conflicts, the determination of the nature of the conflict by a court of law directly impacts on the criminal responsibility of the accused for war crimes. As this article shows, the approach traditionally used in international criminal law to characterise mixed conflicts is not sufficiently reliable for the addressees of penal norms. Low-ranking perpetrators have generally little chance of being aware of the international character of the conflict. This article argues that the principle of individual guilt requires establishing the mens rea regarding the nature of the conflict. Accordingly, a mistake of fact, or alternatively, a mistake of legal element should be admissible defences. Furthermore, it is submitted that both the method of conflict characterisation and the definition of the corresponding mental element are not compatible with the requirements of the principle of legality.