The International Military Tribunal at Nuremberg stated that “a war of aggression… is the supreme international crime… in that it contains within itself the accumulated evil of the whole.”
Justum bellum theory is separated into two branches – Jus ad Bellum and Jus in Bello. There is a paradox here. We can only enforce one or the other. As a result, the idealist Just War theory is basically ignored, while we enforce Laws in War.
State sovereignty includes the right to wage war and there is no higher authority than the state that can say otherwise. States always find justification, true or not, for acting in their self-interest and especially for self-preservation.
According to the Laws of War, states may not wage wars of aggression against foreign states without a just cause. Jus ad bellum exists where War is a last resort, declared by legitimate authorities, waged in self-defense or to restore justice, and fought to bring peace. Just War Theory is derived from Christian doctrines on warfare.
Aggressor states are legally responsible for these conflicts. However, this creates legal distinctions between the two enemy states, producing difficulties in observing and enforcing the Laws in War.
Jus ad Bellum is moot for three important reasons. First, there are no legitimate authorities above the state nor is there a way enforce the law. Second, states rarely admit to engaging in aggressive war and both sides offer justifications. The resulting vagaries make it difficult to discern which party was the true aggressor. In the end, all states claim they fight in “self-defense.” Third, fighting for a just cause opens the door to using any means necessary to win.
Yet Jus ad Bellum continually reappears despite its ineffectiveness or counterproductiveness. Following the First World War, states created a series of treaties and new organizations to redefine the legality of warfare. The League of Nations banned armed forces as a tool of national interests and encouraged the use of peaceful means to settle disputes, such as the new Permanent International Court of Justice or arbitration. The League attempted to create an enforcement mechanism in Article 16 of the Covenant. If a member state committed an illegal act of war against another member state, it would be “an act of war against all Members of the League.” Sanctions and forms of retaliation would restore peace. The League declared that “aggressive war is an international crime.” Subsequently, the Pact of Paris in 1928 renounced the use of war as a tool of national policy. Subsequent to that, the deadliest war in human history.
Under ideal circumstances, international laws could outlaw and prevent aggressive wars, but realistically, since wars occur regardless, it is best to create a legal system to operate within any war and one applied to all belligerents equally and fairly.
Jus in bello manages the conduct of militaries once any armed conflict begins. The Law in War treats all combatants equally regardless of “just causes.” Humanitarian law reinforced the position of jus in bello in order to minimize human suffering during armed conflicts and ultimately, jus in bello became more useful than jus ad bellum.
The Laws in War were original military codes and regulations. Modern laws in war began with the 1863 US Lieber Code during the US Civil War. Soon, most major European states codified standards of behavior for soldiers under their command. There were definition and coordination problems – How does one define POWs, how should they be treated, how should they be exchanged, etc. The Europeans created the Geneva Conventions to standardize existing military codes.
The Codes and humanitarian laws emphasize proportionality and discrimination. They are not to use excessive force, and they must respect the rights of prisoners of war, the wounded, and civilians, which must be reciprocated by the enemy.
Laws in War are also flexible and pragmatic, otherwise they would be inapplicable to actual conflicts. For example, treating POWs well encourages the enemy to surrendered more often, and this lets you get more intelligence out of POWs. No state can live up to the ideal of humanitarian conduct in warfare, but they can minimize unnecessary, self-destructive and unprofessional behavior.
There are exceptions for military necessity and uncertainties in conflict. Militaries have responsibility to follow the laws in war, but if new strategies or technology mean that the old laws are now obsolete, they may discard them. Military necessity was applied to submarine warfare for example.
Legal conduct in an “illegal war” is perhaps a paradox, but since wars continued despite idealists pronouncements against them, creating laws and honor codes to minimize suffering of non-combatants is a reasonable compromise.
In my opinion, the concept of a just war is nonsensical. It is founded in idealist philosophy rather than empirical research of reality. The Laws in War are really military regulations and codes of honor which are much more pragmatic and useful.