War as Limited Conflict

Nor were wars throughout history primarily about killing as many opponents as efficiently as possible. Soldiers sometimes resisted new weapons on the grounds that they were too deadly. War was not necessarily a struggle between good and evil but a decision in a dispute, leaving aside cases involving the abduction of women and enslavement.

The Thirty Years’ War between 1618 and 1648 still shapes collective memory, particularly in Germany. The greatest enemies of both population and soldiers were not combat operations in the narrow sense but looting, disease and famine. The experience of Germany becoming a battlefield for conflicts among foreign powers remains central to the German image of war. With the First World War, the category of guilt was introduced, adding a moral dimension to conflict. That development will be addressed later.

The period after the Thirty Years’ War appears almost curious. The era of cabinet wars, maneuver warfare and guerres en forme formed a bright contrast to the all-consuming war that had preceded it. Mercenary bands were gradually replaced by well-trained standing armies. Soldiers wore elaborate uniforms, and rulers even wondered whether such splendid forces should be risked in battle. Campaigns were no longer sustained by systematic plunder but by magazines and depots. Because armies depended on logistics, cutting supply lines through maneuver often proved decisive.

The opponent was regarded as a just enemy, the justus hostis, who also had a right to seek decision in battle. Criminalizing the adversary was avoided.

Legitimate Causes for War

A reflection of this thinking appears in the work of Dutch jurist and philosopher Hugo Grotius. In his 1625 treatise De Jure Belli ac Pacis Libri Tres, he argued that war was legitimate only for defense, recovery of property or punishment. Territorial expansion or suppression of another people were not considered legitimate aims. A strict distinction was to be made between combatants, identifiable by uniforms, and civilians. Only soldiers were legitimate targets.

In short, there existed a ius ad bellum according to which it was justified to defend legitimate interests such as territory and security by military means.

The First World War marked a break with that perspective, particularly through the attribution of guilt to the German Reich. The principle of moderation and recognition of the enemy gave way to moralization. The logic of limited war, which aimed to leave the opponent a path to surrender, eroded. Moralization meant that the opposing side came to embody evil and therefore had to be destroyed.

The Kellogg–Briand Pact, also known as the Pact of Paris, was signed by 15 states on 27 August 1928. It sought to outlaw war as an instrument of policy, though with limited effect. Aggressive war was condemned, while self-defense remained legitimate. The distinction left many questions unresolved. Each side since then has tended to present its military actions as defensive. Debates surrounding Gaza and Israel, or the Ukraine–Russia front, illustrate how contested the issue remains.

Russia has waged what is officially described since 2022 as a war of aggression against Ukraine. Yet what about the violation of legitimate Russian interests through the expansion of Western influence toward Ukraine, including a prospective EU accession? What about the treatment of Russian-oriented and Russian-speaking populations in the south and east of the country? The situation is not as clear-cut for everyone as the term war of aggression suggests. The moralization of the conflict tends to make it endless. If Russia is cast as absolute evil in this confrontation, negotiations with Vladimir Putin become impossible. Diplomacy, however, requires acknowledging that both sides pursue interests they regard as legitimate. The dichotomy of good versus evil ties the hands of diplomacy.

When War Becomes a Moral Crusade

Moral arguments and appeals to higher values have repeatedly overridden rules that defined war as a contest between equals. When the opponent becomes the absolute enemy, the objective shifts toward destruction. Such wars are often justified as wars to end all wars. Germany is well acquainted with calls for total war. Yet the question remains which side is to be destroyed.

The moralization of war has also had distinctly ugly consequences. Winston Churchill is praised for helping defeat Hitler, yet he also sought the destruction of Germany as a potential competitor and cooperated with Joseph Stalin, who was hardly a humanitarian.

Are Israeli and US strikes on Iran contrary to the law of war? The question currently occupies political commentary and numerous self-appointed legal experts. The claim is difficult to sustain if one takes into account that Iran has for decades financed organizations aimed at Israel’s destruction, including Hamas, Hezbollah, the Houthis and Palestinian Islamic Jihad. Once that is acknowledged, it also becomes easier to understand why Egypt is unwilling to accept Palestinians and why surrounding Arab states show little inclination to defend Iran’s leadership against Israel and the United States.

Democracy Promotion Is Not a Cause for War

It is not a legitimate war aim to "make the world safe for democracy", as US President Woodrow Wilson declared on 2 April 1917 when the United States entered the war against Germany. He justified the step by arguing that Berlin had decided "to put aside all restraints of law or of humanity" in its submarine campaign. Such sweeping slogans should invite skepticism. The liberation of Iranians from the ruling clerical establishment is likewise not a legitimate war aim. That task would have to be undertaken internally. Eliminating a persistent threat to Israel, however, can be regarded differently.

US attempts at regime change in Iraq, Afghanistan and Libya are not remembered favorably. They failed and proved destabilizing. Libya offers a particularly clear example. Muammar Gaddafi had long been tolerated before being recast as a villain. The fear that the United States could again fall into a similar trap is understandable. Donald Trump had promised a different course. The United States is therefore walking a fine line.

Is the law of war therefore only a paper tiger that allows the militarily stronger side to do anything? No. Everything that resists the moralizing dichotomy of final good versus evil preserves the option of diplomacy. That may be uncomfortable for those convinced that higher insight allows them always to distinguish clearly between good and evil. Periods in which belligerents agreed on rules aimed at decision rather than destruction were, even in war, the better times.

The debate about the law of war and its defense is therefore worthwhile. The paper tiger still has a few teeth.