The origins of Just War Theory

Ethical questions of war can be traced back to classical antiquity and across the histories of all the main civilisations. Just War Theory is a set of concepts and values that have been refined throughout its history. There have been some key figures throughout the period whose contributions to the theory have shaped what it has become today.

There are two clearly defined elements to the theory:

1. Jus ad bellum – When is it just to go to war? And,
2. Jus in bello - When we are in war, how and who should we fight?

There is also an emerging third element of the theory: Jus post bellum – what is ethical conduct in the aftermath of war?

But for a very short history lesson, Just War Theory can be traced as far back as Greek philosophers Plato and his student Aristotle (3^rd or 4^th century BC).

Aristotle is credited with coining the term ‘Just war’ in contrast to the earlier idea of a ‘holy war’ as mandated by God.

Just War Theory is today identified as a secular concept but its values are based on the origins of the Greeks and Romans, as well as Christian values.

And we will see that those Christian values played a significant role in developing the theory in the 5^th Century AD when St Augustine was asking himself and his fellow Christians whether they could engage in war without sin. St Augustine is often credited with being the founder of the Theory, but in fact there was much that came before him.

It was from Aristotle, that we first see the reference to what is now considered by Just War theorists as the most obvious just cause for war: Self defense. He thought it morally justified to go to war to prevent one’s community from being attacked and enslaved by another. But we may be uncomfortable today with his other justifications for war.

And we can see how over time different concepts for Just War have been considered, but rejected. Aristotle thought it ok to go to war for empire expansion provided:

1. The empire would benefit everyone, including the conquered
2. The empire would not be so large and rich that it would attract attacks and therefore create more wars
3. He also thought it was also morally acceptable to go to war to collect for  slaves, as long as the slaves were naturally submissive.


A little later came the Romans, and amidst their shattering success on the battlefields as their empire expanded – Roman emperors and Senators developed some of the deepest reflections on the ethics of war.

Roman lawyer Cicero (106-43 BCE) built further on Aristotle’s early development of the theory. But he rejected the need for more slaves as a just cause for war. He also added the rules that are familiar concepts of the theory today: proper authority, public declaration and war as a last resort.

It was St Augustine, 500 years later, who was wrestling with the dilemma of how to be both a good Christian and a public official of the warring Roman Empire, who contributed the thinking around the right intentions for going to war.

Shouldn’t he as a Christian, always take the Pacifist position: one must always show love and non-violence? But also, isn’t protecting his own people from aggression not showing love for them? 

To help develop this, Augustine reconciled this with incorporating ‘Right Intention’ into the Just War Theory. He insisted the intention of war must be love for and the desire to protect the endangered innocents, and without any joy from the bloodlust itself, therefore good men can undertake wars in the obedience of god as long as their intention is for the love and protection of their own.

Interestingly here, Augustine did not reject the connotation that a Holy War was just cause for war and in doing so, according to Brian Orend, allowed for a blurring of the lines on this matter that would last for another 1000 years within which time the Pope-ordered crusades would take place.

Within those 1000 years, the Roman Empire had collapsed in the West, and the Dark Ages took hold with the only institution left standing the Catholic Church. This was a period of messy, private, feudal wars, with the Crusades coming towards the end of this period.

Despite the Theory having now been in some form of existence for roughly 1500 years at this point, this is considered a period of regression for Jus ad bellum – the just cause of war – due largely to the muddling of what was declared holy war and/or a just war.

The Theory did still have some notable bright moments. In this time the Church led peace movements to protect innocent people, particularly women and children caught in the crossfire. Coined the “Peace of God’, this concept would later form what is widely considered the most important element of Jus in bellum – justice in war – or ‘non-combatant immunity’.

What’s the difference between a Holy war and a just war?

The difference between the two is in short: A holy war is a war considered approved of by God, or by a direct messenger of God, for example the Pope – in contrast a just war is not sacred, but moral.

Italian theologian Thomas Aquinas around the time of the last crusade questioned whether a holy war was just, saying that defensive wars protecting Christians from death or persecution at the hands of non-Christians might be permissible but aggressive wars designed to coerce non-believers into Christianity were not.

It would still take another two centuries before there would start to be clear definitive proclamations against the use of holy wars as a justification for war.

In the 16^th Century, the theory was being effectively used not only to test the legitimacy of the decision to go the war, but also to criticize the powers that authorized the war, urging them to stop the fight. Two legal figures, one Spanish and one Dutch, were both using Just War Theory to criticise their own nation’s justification and conduct in the wars they waged.

Spanish jurist Vitoria, living in the time of the Spanish Conquest, heavily criticized the conduct of the Conquistadors, using the theory to largely resolve wars of conquest were unjust. Spain was motivated neither by love – the right intention Augustine said was required, nor by any self-defense or protection, but were driven by greed and love of power.

Vitoria was the first to say clearly that non-Christian communities have the right not to be attacked and enslaved. He rejected the concept of holy wars and insisted on the secularism of the Just War Theory.

His work was responsible for extending what had been up til now a set of guidelines for Christian princes on how to conduct war in Christian Europe to universal principles – or natural law applicable to anyone, anywhere at anytime.

One of his most important contributions was his stress on the quality and objectiveness of the evidence needed to support the decision to go to war.

We can fast-forward here 500 years and apply this concept to President George W. Bush in his 2003 State of the Union address, which ‘justified’ the impending Iraqi invasion in just ’16 words’:

"The British government has learned that Saddam Hussein recently sought significant quantities of uranium from Africa”

Evidence we now know was vehemently rejected:

Dutch theologian and lawyer Hugo Grotius was also deeply distressed by the behaviour he was seeing from his fellow countrymen in their colonization, particularly through the Caribbean Indies, as well as what the Wars of Religion, between Catholics and Protestants – across Europe. He sought to strongly reinforce Vitoria’s rejection of Holy Wars, the combination of which sealed the debate on the issue in the Just War Tradition.

Grotius’s work is also credited as one of the first productions of the ‘laws of armed conflict’. Within 200 years after Grotius’s work, the moral principles of war were being translated into specific legal codes: and international treaties and laws regulating armed conflict culminated in the Hague Conventions between 1899 and 1907.

The question for today is whether the theory is still relevant?

Jeff McMahan argues it is no longer relevant, due to the changing character of war – wars are not always between two clear sides on the battlefield today and the state-centric framework of the traditional theory can no longer apply.

He is part of a new school of revisionist theorists, challenging the traditional theory – asserting the principles of jus ad bellum and jus in bello cannot be so clearly separated and that the principles of jus ad bellum apply to the individual soldier, not just the state.

Michael Walzer, however, is the 20^th century’s most influential defender of the Theory.

Walzer on the origins of Just War Theory and the development jus post bellum:

 Janna