Throughout history, whenever states and/or peoples have taken up arms, they have asserted that they were doing so for a just cause. All too often this argument has been used to justify refusing their opponents any mercy. In fact, history shows that the more the belligerents insist on the sanctity of their reasons for resorting to armed force, the more those same reasons are used to justify the worst excesses. The crusades and the wars of religion, alas, left a long trial of atrocities in their wake.

It was only when war was recognized as a very imperfect means of settling a dispute between two sovereigns that states began to accept the idea of limiting armed violence. The emergence of nation states and the development of professional armies led states to gradually accept a body of rules intended to limit the horrors of war and to protect its victims. For a long time, these rules remained customary in nature; they began to be codified in the mid-Nineteenth Century.

International Humanitarian Law developed at a time when the use of force was a lawful form of international relations, when states were not prohibited to wage war, when they had the right to make war, meaning, when they had the Jus ad bellum. There was no logical problem for international law to prescribe them the respect of certain rules of behavior in war called the jus in bello if they resorted to that means.

Today the use of force between states is prohibited by a peremptory rule of international law. This has made the jus ad bellum to change into a jus contra bellum. Exceptions to this prohibition are admitted in case of individual and collective self-defense, Security Council enforcement measures, and arguably to enforce the right of peoples to self-determination or national liberation wars. Logically, at least one side of an international armed conflict is, therefore, violating international law by the sole fact of using force, however respectful of IHL. All municipal laws of the world equally prohibit the use of force against governmental law enforcement agencies in the case of non-international armed conflict.

Although armed conflicts are prohibited, they happen, and it is today recognized that international law has to address this reality of international life not only by combating the phenomenon, but also by regulating it to ensure a minimum of humanity in this inhumane and illegal situation. However, for practical, policy, and humanitarian reasons, international humanitarian law has to be the same for both belligerents: the one resorting lawfully to force and the one resorting unlawfully to force. From a practical point of view, the respect of international humanitarian law could otherwise not be obtained, as, at least between the belligerents, it is always controversial as to which belligerent is resorting to force in conformity with the jus ad bellum and which violates the jus contra bellum. In addition, from a humanitarian point of view, the victims of the conflict on both sides need the same protection, and they are not necessarily responsible for the violation of the jus ad bellum committed by “their” party.

International Humanitarian Law has, therefore, to be respected independently of any argument of jus ad bellum and has to be completely distinguished from jus ad bellum. Any past, present, and future theory of just war only concern jus ad bellum and cannot justify that those fighting a just war have more rights or less obligations under international humanitarian law than those fighting an unjust war.

This complete separation between jus ad bellum and jus in bello has been recognized in the preamble of protocol I which reads:

“The High Contracting Parties,

 Proclaiming their earnest wish to see peace prevail among          

peoples, Recalling that every state has the duty, in conformity with the charter of the United Nations, to refrain in its international relations from the threat or use of force against the sovereignty, territorial integrity or political independence of any state or in any other manner inconsistent with the purposes of the United Nations

Believing it necessary nevertheless to reaffirm and develop the provisions protecting the victims of armed conflicts and to supplement measures intended to reinforce their application,

   Expressing their conviction that nothing in this protocol or in the Geneva Conventions of 12 August 1949 can be construed as legitimizing or authorizing any act of aggression or any other use of force inconsistent with the charter of the United Nations,

Reaffirming further that the provisions of the Geneva Conventions of 12 August 1949 and of this protocol must be fully applied in all circumstances to all persons who are protected by those instruments without any adverse distinction based on the nature or origin of the armed conflict or on the causes espoused by or attributed to the parties to the conflict. (...)”

This complete separation between jus ad bellum and jus in bello implies that international humanitarian law applies whenever there is de facto armed conflict, however that conflict can be qualified under jus ad bellum, and that no jus ad bellum arguments may be used in interpreting international humanitarian law. It also, however, implies, for the drafting of rules of international humanitarian law, that they may not render the Jus ad bellum impossible to be implemented, e.g., render efficient self-defense impossible. 

There are also some writers who do not confine themselves to just showing the distinction between jus ad bellum and jus in bello and who go further and even assert the autonomy of jus in bello with regard to jus ad bellum. Under the terms of the Pact of Paris, the contracting states declared that they condemned ‘recourse to war for the solution of international controversies’, and renounced it ‘as an instrument of national policy’. As has been noted herein above the United Nations Charter has prohibited any recourse to force in international relations with few exceptions.

That being the case, the following question arises: Is the fact that a belligerent has resorted to armed force in violation of international treaties and commitments an obstacle to the application of jus in bello? Two possibilities may be envisaged:

    I)  Either the war of aggression is deemed to be the international crime par excellence, a crime which subsumes all others and which therefore cannot be regulated, in which case the laws and customs of war do not apply to either of the belligerents; or

    II)  The aggressor alone is deprived of the rights conferred by jus in bello, whereas all his obligations under this law remain unchanged, while the state which is the victim of the aggression continues to enjoy all the rights conferred by jus in bello without incurring any obligations.

The first hypothesis is only one that draws all the logical conclusions from any subordination of jus in bello to jus ad bellum. It must nevertheless, be rejected out of hand, for it would lead to unbridled violence. The consequence of an abdication of the rule or law, that solution would produce absurd and monstrous result.

The second solution entails a differentiated application of the laws and customs of war, but it must be rejected just as vigorously as the first, for in practice it would produce the same result. In the absence of a mechanism to determine aggression and to designate the aggressor in every case and in such a way as to be binding equally all belligerents, each of the latter would claim to be the victim of aggression and take advantage of this to deny his adversary the benefits afforded by the laws and customs of war. In practice, therefore, this solution would lead to the same result as the hypothesis whereby wars of aggression cannot be regulated: a surge of unchecked violence. The autonomy of jus in bello with regard to jus ad bellum must therefore be preserved. This conclusion had already been clearly demonstrated by Emer de Vattel (1714-1767):  

            War cannot be just on both sides: One party claims a right, the other disputes the justice of the claim; one complains of an injury, the other denies having done it. When two persons dispute over the truth of a proposition it is impossible that the two contrary opinions should be at the same time true. However, it can happen that the contending parties are both in good faith; and in a doubtful cause it is, moreover, uncertain which side is in the right. Since, therefore, Nations are equal and independent, and can not set themselves up as judges over one another, it follows that in all cases open to doubt the war carried on by both parties must be regarded as equally lawful, at least as regards its exterior effects and until the cause is decided.

Thus, Vattel does not expressly reject the doctrine of just war, developed by the fathers of the Church, but puts it into perspective and draws its sting.

The autonomy of jus in bello with regard to jus ad bellum was confirmed after the Second World War by the Charter of Nuremberg Tribunal, which made a distinction between war crimes, that is, acts committed in violation of the laws and customs of war, and crimes against peace. This distinction was confirmed by the practice of the Tribunal. Indeed, the Tribunal scrupulously respected the distinction between crimes against peace, on the one hand, and war crimes, on the other; it assessed the intrinsic unlawfulness of war crimes against the laws and customs of war, regardless of the fact that the crimes concerned had been committed during a war of aggression. By acknowledging that the laws and customs of war could be invoked not only by the prosecution but also by the defense for the accused, the Tribunal unequivocally confirmed the autonomy of jus in bello with regard to jus ad bellum. The great majority of national tribunals entrusted with the task of judging war crimes committed during the Second World War upheld this distinction.

The Geneva Conventions of 12 August 1949 doubly confirmed the autonomy of jus ad bellum. First, in Article 1 common to the four Conventions, the High Contracting parties undertake to respect and ensure respect for these instruments ‘in all circumstance.’ There can be no doubt that in adopting this provision states ruled out the possibility of invoking arguments based on the legality of the use of force in order to be released from their obligations under the Conventions.

Secondly, the Conventions prohibit any reprisals against persons or property protected by their provisions. Obviously, any state using the argument that it is the victim of a war of aggression to justify its refusal to apply humanitarian law to enemy nationals would be in violation of this prohibition.

Finally, the preamble to Protocol I additional to the Geneva Conventions, adopted by consensus on 7 June 1977, put an end to all argument on the matter by a pointing out that:      

... The provisions of the Geneva Conventions of 12 August 1949 and of this protocol must be fully applied in all circumstances to all persons who are protected by those instruments, without any adverse distinction based on the nature or origin of the armed conflict or on the causes espoused by or attributed to the Parties to the conflict.

The principle of the equality of belligerents before the law of war, which is in a way the corollary of the autonomy of jus in bello with regard to jus ad bellum, is thus firmly rooted in both treaty law and state practice.

This principle dominates the entire body of the laws and customs of war. It finds its main application, however, in the status of prisoners of war as it took shape in Europe from the Seventeenth Century. The decision to make war was the responsibility of the sovereign alone; the soldier who was in the sovereign’s service could not be held responsible for his participation in the hostilities. Hence, captivity in a war situation was no longer seen as a dishonor or a punishment but as a security measure whereby the captor prevented enemy against him. When peace was restored, prisoners of war had to be freed, regardless of their number or rank and without any ransom being demanded. This was the rule laid down by Article LXIII of the Treaty of Munster of 30 January 1648, which put an end to the Thirty Years War. ‘All Prisoners of War shall be released on both sides, without payment of any ransom, without distinction and without exception...’  

If the application of the principle of the equality of belligerents before the law of war raises major difficulties in situations of international armed conflict, it may well be imagined that even more formidable obstacles lie in its way in situations of non-international armed conflict. Indeed, a state facing an insurrection will almost invariably begin by invoking a dual inequality: On the one hand, the state will accuse the insurgents of having violated national law and endeavour to bring the full force of the criminal law to bear against them; while claiming to be fully within its rights, it will do everything it can to criminalize its adversaries; On the other hand, the state will rely on the inequality of the insurgents’ legal status under domestic law and, in most cases, under international law, to justify rejecting any relationship with them based on an equal footing.

This clearly indicates the case where by the autonomy of jus in bello with regard to jus ad bellum and the principle of the equality of belligerents before the law of war meet with particular obstacles in situations of non-international armed conflict.