What is a ‘Just War’?

On the 20th anniversary of the worldwide protest against the Iraq War in February 2003, we ask: is there such a thing as ‘just war’ or is there just war?

Painting by F.J. Mears, before 1930. Wiki Commons.

‘Does it matter all that much how we break things and make our enemy’s mothers weep?’

Cathal J. Nolan, Professor of History at Boston University and author of Mercy: Humanity in War (Oxford University Press, 2023)

During the interminable wars among the city states of Renaissance Italy, a contract captain (condottieri) leading a mercenary company had engraved on his breastplate: ‘Enemy of God, Enemy of Piety, Enemy of Pity.’ It is sentiments such as these that led Niccolò Machiavelli to reject the Augustinian and medieval ideal of the ‘just war’, arguing, instead, that there is just war. Machiavelli knew that princes did not act inside a system of law or moral constraint. They lived or went extinct in naked competition for power, cloaked in talk of higher law. Warrior popes sputtered hypocrisies about universal morality, that the laws of God were superior to the will of men. Yet the princes did what they chose. As Machiavelli put it: ‘War is just when it is necessary.’ If he was right, and it is hard as a military historian to say he was not, the only just wars are defensive. No war of aggression can be just, which is why we devote so much effort to saying the other side started it. Quite often, neither side’s cause is just. There is just war.

Fighting abrades every ideal of moral and legal restraint. This is a cardinal truth. We must not be distracted by abstract debate over whether some lawyerly rule drafted in peacetime, or a weapon, is more-or-less moral. We have banned dum-dum bullets and poison gases, corrosive chemicals, spike pits, bio toxins, plastic landmines and cluster bombs. But once a war begins, does it matter all that much how we break things and make our enemy’s mothers weep? Beyond a right of self-defence, it is hard as a historian to credit claims to morality or justice in the means by which war is waged. We can agree that Nazi Germany had to be stopped, but we should admit that some means used, while arguably necessary, were not absolved by the Allied cause: mass starvation (food blockade) and targeting civilians (morale bombing).

War is an arena of extraordinary threat and extreme moral conditions. It is not a seminar room or a courthouse where the ethics or law of hard cases are to be argued out.


‘To be “just”, a war must be fought according to recognised rules of conduct’

Peter H. Wilson, Chichele Professor of the History of War at All Souls College, Oxford and author of Iron and Blood: A Military History of the German-speaking Peoples Since 1500 (Allen Lane, 2022)

The justice of a particular conflict is invariably as contested as the belligerents’ motives for fighting, with the victors usually asserting their interpretation in any peace settlement. Theologians and lawyers have long disagreed over definitions of what constitutes a ‘just war’, but Europe’s imperial dominance during the 19th century led to the medieval Christian tradition being imposed globally through the codification of modern international law in the Geneva and The Hague conventions.

This holds that, to be ‘just’, a war must be fought according to recognised rules of conduct, such as treating prisoners humanely and avoiding prohibited weapons. War can only be waged legitimately by a ‘recognised authority’ possessing a ‘just cause’, generally defined as redressing some wrong committed by the other side. Finally, resort to arms must pursue the ‘right intention’ of promoting good while combating evil.

The definition of what constituted a ‘recognised authority’ narrowed progressively by about 1815 with the emergence of the sovereign state monopolising legitimate violence while delegitimating previous practices, such as privateering or mercenary service. This process culminated in the international order enshrined in the League of Nations in 1920 and its successor, the United Nations, in 1945. Both these institutions recognised only sovereign states as legitimate warmakers.

Only 17 states have formally declared war (or a state of war) since 1948, with most presenting their use of armed force as ‘actions short of war’ to reduce the risk of international sanctions or intervention by other powers. Meanwhile, the lines between classic war and other forms of conflict have become blurred, particularly since the end of the Cold War, through a combination of the growing fragility of many states and the inability of even relatively powerful ones to control private military and security companies.

History cannot solve these problems but it does remind us that the current definitions of ‘just war’ are themselves the product of a contested past.


‘Between 1943 and 1945 three Italian armed forces operated – who could claim jus ad bellum?’

Vanda Wilcox, Author of The Italian Empire and the Great War (Oxford University Press, 2021)

The Italian constitution ‘repudiates war as an instrument of aggression against the freedoms of other peoples and as a means for settling international controversies’. Instituted in 1948, the new republic’s constitution was a reaction to the traumatic experience of fascism and Italy’s unjust wars of aggression against Ethiopia, Greece and Yugoslavia (among others). Its embrace of just war theory closely echoed the UN Charter. But, given that the republic was born from the Resistance of 1943-45, it also makes an implicit judgement that this was itself a just war.

Between 1943 and 1945 three Italian armed forces operated: Mussolini’s re-founded Italian Social Republic (RSI), the Italian Royal Army, which fought alongside the Allies, and the partisan forces of the Resistance. Who could claim jus ad bellum? The RSI’s claims rested on its assertion of sovereignty: if it were indeed a legitimate state, it had the right of self-defence against the Allied invaders. However, since the war arose from Italy’s illegitimate invasions of 1940, Mussolini’s war could never be just. The Italian Royal Army’s situation was simpler: king and government having declared co-belligerency alongside the Allies, it was fighting a proportionate war against a usurper of state power (Mussolini) and his foreign backers (the Nazi regime).

Most complex was the case of the Resistance. Their cause was clearly just, and taking up arms against fascism was certainly a last resort, thus satisfying two important requirements of just war theory. But what of right authority? Can any force in a civil conflict claim it?

In September 1943 representatives of multiple political parties united in Rome to create the National Liberation Committee, to coordinate the military resistance to Nazism and fascism. This was also a political force, the basis of the new Italian government, which aimed to serve as an authority in whose name acts of war might be undertaken. The Resistance’s war aimed to create the state that would then legitimise it. It thus mirrored the 19th-century wars of Italian unification, which also strove to bring into being the right authority to justify their war – a paradoxical kind of just war.


‘How could one prosecute jihad against fellow Muslims? There was always a way’

Justin Marozzi, Author of Islamic Empires: Fifteen Cities that Define a Civilization (Allen Lane, 2019)

Islam started as an act of conquest in the seventh century, first within and then without the Arabian Peninsula, so perhaps it should be no surprise that legitimating war came easily to the new faith.

Starting with Imam Abu Hanifa, the eighth-century Iraqi scholar, early Muslim jurists conceived of the world in Manichean terms, dividing it into two opposed realms: The Dar al-Islam, in shorthand the Muslim world, and the Dar al-Harb, the House of War.

Though these terms do not appear in the Quran, the holy book licenses conquest and plunder under the rubric of holy war, jihad, a phenomenon whose iterations can be traced from the martial life of the Prophet Mohammed in the seventh century to so-called Islamic State fighters in the 21st. Muslims are enjoined to take up the fight and ‘consume the booty that you have captured as lawful benefit’.

In the broadest terms, then, war against the infidels is just by definition. Not only that, it is also a responsibility, as the conquering Rashidun, Umayyad and Abbasid caliphs of the first centuries of Islam understood. As with any holy book, however, interpretations differ widely. Some scholars believed war was only legitimate in self-defence, others argued the ‘sword verses’ of the Quran justify war against infidels as an essential tool for the spreading of Islam.

Muslim jurists, like those the world over, knew which side their bread was buttered. In 1570, when the Ottoman Sultan Selim II wanted to invade the Venetian island of Cyprus in contravention of a treaty, he turned to Ebussuud Efendi, his obliging Sheikh-ul-Islam, the supreme judge of the empire. Was it legitimate to break the treaty? Yes, came the answer, a treaty with the infidels was only legal when it benefited all Muslims. Otherwise it was ‘absolutely obligatory and binding’ to break it.

And Muslim rulers, like their Christian counterparts, have often proved happy to break the rules whenever it suited them. How could one prosecute jihad against fellow Muslims? Easy. In 1398, for example, Timur, or Tamerlane, justified his invasion of the Muslim sultanate of Delhi by arguing its rulers had been ‘slack in their defence of the Faith’. There was always a way.