‘Just War’ Theory: Its Endurance Through the Ages

The Illustrious Hugo Grotius of the Law of Warre and Peace: 
With Annotations, III Parts, and Memorials of the Author’s Life and Death.
Book with title page engraving, printed in London, England, by T. Warren for William Lee in 1654.

Posted by Keith Tidman

To some people, the term ‘just war’ may have the distinct ring of an oxymoron, the more so to advocates of pacifism. After all, as the contention goes, how can the lethal violence and destruction unleashed in war ever be just? Yet, not all of the world’s contentiousness, neither historically nor today, lends itself to nonmilitary remedies. So, coming to grips with the realpolitik of humankind inevitability waging successive wars over several millennia, philosophers, dating back to ancient Greece and Rome — like Plato, Aristotle, and Cicero — have thought about when and how war might be justified.

Building on such early luminary thinkers, the thirteenth-century philosopher and theologian Saint Thomas Aquinas, in his influential text, Summa Theologica, advanced the principles of ‘just war’ to a whole other level. Aquinas’s foundational work led to the tradition of just-war principles, broken down into jus ad bellum (the right to resort to war to begin with) and jus in bello (the right way to fight once war is underway). Centuries later came a new doctrinal category, jus post bellum (the right way to act after war has ended).

The rules that govern going to war, jus ad bellum, include the following:

• just authority, meaning that only legitimate national rulers may declare war;

• just cause, meaning that a nation may wage war only for such purposes as self-defence, defence of other nations, and intervention against the gravest inhumanity;

• right intentions, meaning the warring state stays focused on the just cause and doesn’t veer toward illegitimate causes, such as material and economic gain, hegemonic expansionism, regime change, ideological-cultural-religious dissimilarities, or unbridled militarism;

• proportionality, meaning that as best can be determined, the anticipated goods outweigh the anticipated evil that war will cause;

• a high probability of success, meaning that the war’s aim is seen as highly achievable; 

and...

• last resort, meaning that viable, peaceful, diplomatic solutions have been explored — not just between potentially warring parties, but also with the intercession of supranational institutions, as fit — leaving no alternative to war in order to achieve the just cause.

The rules that govern the actual fighting of war, jus in bello, include the following: 

• discrimination, meaning to target only combatants and military objectives, and not civilians or fighters who have surrendered, been captured, or are injured; 

• proportionality, meaning that injury to lives and property must be in line with the military advantage to be gained; 

• responsibility, meaning that all participants in war are accountable for their behaviour; 

and... 

• necessity, meaning that the least-harmful military means, such as choice of weapons, tactics, and amount of force applied, must be resorted to.

The rules that govern behaviour following war’s end, jus post bellum, typically include the following: 

• proportionality, meaning the terms to end war and transition to peace should be reasonable and even-handed; 

• discrimination, meaning that the victor should treat the defeated party fairly and not unduly punitively; 

• restorative, meaning promoting stability, mapping infrastructural redevelopment, and guiding institutional, social, security, and legal order; 

and... 

• accountability, meaning that determination of culpability and retribution for wrongful actions (including atrocities) during hostilities are reasonable and measured.

Since the time of the early philosophers like Augustine of Hippo, Thomas Aquinas, and the ascribed ‘father of international law’ Hugo Grotius (The Law of War and Peace, frontispiece above), the principles tied to ‘just war’, and its basis in moral reciprocity, have shifted. One change has entailed the increasing secularisation of ‘just war’ from largely religious roots.

Meanwhile, the failure of the seventeenth-century Peace of Westphalia — which ended Europe’s devastating Thirty Years’ War and Eighty Years’ War, declaring that states would henceforth honour other nations’ sovereignty — has been particularly dreadful. As well intentioned as the treaty was, it failed to head off repeated militarily bloody incursions into others’ territory over the last three and a half centuries. Furthermore, the modern means of war have necessitated revisiting the principles of just wars — despite the theoretical rectitude of wars’ aims.

One factor is the extraordinary versatility, furtiveness, and lethality of modern means of war — and remarkably accelerating transformation. None of these ‘modern means’ were, of course, even imaginable as just-war doctrine was being developed over the centuries. The bristling technology is familiar: from precision (‘smart’) munitions to nuclear weapons, drones, cyber weapons, long-range missiles, stealthy designs, space-based systems, biological/chemical munitions, global power projection by sea and air, hypervelocity munitions, increasingly sophisticated, lethal, and hard-to-defeat AI weapons, and autonomous weapons (increasingly taking human controllers out of the picture). In their respective ways, these devices are intended to exacerbate the ‘friction and fog’ and lethality of war for the opponent, as well as to lessen exposure of one’s own combatants to threats. 

Weapons of a different ilk, like economic sanctions, are meant to coerce opponents into complying with demands and complying with certain behaviours, even if civilians are among the more direly affected. Tactics, too, range widely, from proxies to asymmetric conflicts, special-forces operations, terrorism (intrinsically episodic), psychological operations, targeted killings of individuals, and mercenary insertion.

So, what does this inventory of weapons and tactics portend regarding just-war principles? The answer hinges on the warring parties: who’s using which weapons in which conflict and with which tactics and objectives. The idea behind precision munitions, for example, is to pinpoint combatant targets while minimising harm to civilians and civilian property.

Intentions aren’t foolproof, however, as demonstrated in any number of currently ongoing wars. Yet, one might argue that, on balance, the results are ‘better’ than in earlier conflicts in which, for example, blankets of inaccurate gravity (‘dumb’) bombs were dropped, and where indifference among combatants as to the effects on innocents — impinging on noncombatant immunity — had become the rule rather than the exception.

There are current ‘hot’ conflicts to which one might readily apply just-war theory. Yemen, Somalia, Libya, Syria, Ukraine, India/Pakistan, Iraq, and Afghanistan, among sundry others, come to mind. (As well as brinkmanship, such as with Iran, North Korea, and Venezuela.) The nature of these conflicts ranges from international to civil to terrorist to hybrid. Their adherence to jus ad bellum and jus in bello narratives and prescriptions differ radically from one to another. These conflicts’ jus post bellum narratives — meaning the right way to act after war has ended — have still to reveal their final chapter in concrete treaties, as for example in the current negotiations between the Taliban and United States in Afghanistan, almost two decades into that wearyingly ongoing war. 

The reality is that the breach left by these sundry wars, either as they end abruptly or simply peter out in exhaustion, will be filled by another. As long as the realpolitik inevitability of war continues to haunt us, humanity needs Aquinas’s guidance.

Just-war doctrine, though developed in another age and necessarily having undergone evolutionary adaptation to parallel wars’ changes, remains enduringly relevant — not to anaesthetise the populace, let alone to entirely cleanse war ethically, but as a practical way to embed some measure of order in the otherwise unbridled messiness of war.