just war theory

Just War Theory

29 March 2022

7.1 MINS

Thinking about right, wrong and justice in warfare. Just war theory, as formulated by Christian thinkers down the ages, outlines the principles by which a nation may legitimately enter a war.

In a fallen world, war, the use of force, bloodshed, violence and aggression will always be with us. That — for the Christian — is because sin is always with us. The believer knows that God established civil government to help curb violence and injustice, and maintain a modicum of peace and order.

Thus rulers, judges and the police have delegated authority to use force and the like. This is true of domestic policy as well as of international relations — thus the need for armies. A few initial things can be said about war and peace. One, some Christians have felt that pacifism is the only option for the believer — but that has always been a minority position, with most Christians holding the view that war can sometimes be justified.

Two, we know that war can be biblically justified. Yahweh is a “god of war” for example (Exodus 15:3) and he “trains my hands for battle” (Psalm 18:34). He even gives somewhat detailed instructions on how warfare is to be carried out (see Deuteronomy 20). If all warfare was morally wrong and sinful, the hundreds of warfare and military metaphors used in Scripture would be highly out of place.

Three, the use of legitimate force differs from violence. A parent, a policeman and a soldier can use force — but that is not the same as violence. See more on this distinction here.

As to just war theory, this article will be a very brief introduction to the subject. Much more detailed articles looking more closely at the various issues involved will be forthcoming. I will here just do three things: offer a short historical overview of key thinkers; look at the principles commonly run with; and offer a list of books for further reading.


Just war theory is just that: it is not a set of hard and fast rules, but general principles and thoughts on how war might be fought justly. These thoughts extend to over 2500 years, incorporating classical Greek and Roman philosophers as well as Christian theologians. There are many names that I could mention here, but let me highlight just some of the more important ones.

The Greek philosopher Aristotle (384-322 BC) was one of the earliest to think carefully about the ethics of warfare. And he seems to have been the first to use the phrase “just war”. The Roman statesman Cicero (106-43 BC) was a major thinker in this area, emphasising defensive wars, and wars to right various wrongs.

As I state in the article I linked above, the early church was in no position to be involved in political life. It was a small persecuted minority trying to stay alive, so believers back then were not seeking to hold office or become involved in the military. But when Christianity was no longer a targeted religion in the Roman Empire, Christians then could and did get involved in both politics and the military.

Ambrose (c. 339-397) was one of the earliest Christian thinkers to deal with questions of war and peace. The importance of securing the peace (just intent) was a major concern of his. Augustine (354-430) offered the fullest Christian development of just war thinking up to this point, dealing with a wide range of issues. These and other Christian thinkers dealt with the ethics of warfare utilising both past secular thought as well as biblical data.

Aquinas (1225-1274) was a very important Christian theologian in so many ways, not least of which when it came to issues of politics, war and peace. Like others, he builds heavily on Augustine, but also on Aristotle. He especially emphasised the importance of the “tranquility of order” (see below under ‘just intent’). Defending the common good and upholding a just and ordered society are a big part of just war, he argued.

He also stressed the “principle of double effect” wherein a bad outcome (e.g. the death of others) may be part of a larger, noble aim (to secure the peace and end hostilities). That is, certain acts can have both intentional and unintentional effects. The stated intent may be to end a war, but an unintended effect might be the death of some civilians, and so on.

The Spanish philosopher and theologian Francisco de Vitoria (1485-1546), was also crucial in the development of just war theory, especially as he sought to pull the various strands together into a coherent whole. He stressed issues like legitimate authority, just cause, and proportionality.

One last important figure to briefly note is the Dutch Protestant and father of international law, Hugo Grotius (1583-1645). The Thirty Years’ War (1618-1648) occupied a large part of his intention. These were more than just religious wars, and Grotius discussed how just war thought could inform us concerning the changing political and religious situations taking place. The development of the modern nation-state and the like was the backdrop to his thinking.

One area he focused on was whether a just war must always avoid a first strike. That is, must it always involve a second strike, or is there in fact a place for a preemptive strike, or a preemptive war? He argued that sometimes such a strike can prevent greater carnage, bloodshed and injustice.


As mentioned, these are not fixed laws or rules set in stone — they are general principles that have evolved over the centuries to help us think ethically about warfare. While some — pacifists and the like — might try to argue that ‘just war’ or ‘ethical warfare’ are oxymorons, many of the best thinkers in history — including many of the greatest Christian minds — have thought otherwise.

There are various formulations and lists one can appeal to, but what I offer here is fairly standard. And there have often been two main categories here: jus ad bellum (justice in going to war) and jus in bello (justice while fighting a war). That is, there are principles of right and wrong in declaring a war, and in conducting a war. Here I offer four standard principles of the former, and three of the latter.

Just cause. The basic principle here is that self-defence is the main ethically legitimate reason for going to war. Wars of aggression are not, but of course, both types of wars are not always so easily defined. Can a preemptive strike be used as part of defensive warfare? The idea of defending the innocent, stopping aggression, and restoring justice are key themes here.

Just intent. This follows on from just cause. The morally legitimate use of war is to maintain justice and restore peace. Peace with justice, or a well-ordered peace (Tranquillitas Ordinis) is the aim. The intention is to keep the peace, but a peace where justice is maintained.

Last resort. Peaceful means to end or prevent conflict should be tried first. Jaw jaw is better than war war, as the saying goes. Diplomacy, arbitration and negotiation should first be utilised as far as possible. Of course, often aggressors are not interested in diplomacy, and seeking to end hostilities by means of peace talks and so on can often be too little too late.

Lawful declaration, or competent authority. Generally, it is insisted that only lawful government can declare a war. Individuals or groups within a nation cannot. But obviously, questions arise as to just what a ‘lawful government’ is, and so on. For example, is a dictatorship still considered to be a legitimate authority?

Non-combatant immunity. The aim here — as much as possible — is to avoid civilian casualties. This of course becomes difficult when military targets are deliberately positioned in civilian populations, or ‘human shields’ are used by an enemy. Collateral damage will almost certainly occur in any armed conflict, but the aim is to minimise this as much as possible.

Limited objectives. Usually, unconditional surrender is not to be seen as the ultimate objective of war, but the ending of hostilities and the restoration of peace. Questions arise of course as to how far defensive wars must be fought to bring hostilities and aggression to an end.

Limited means. Generally speaking, only sufficient force should be used. Restraint in the use of weaponry is part of this, and various international conventions have outlawed certain forms of warfare, such as using chemical weapons. The use of military means to get the job done, but not resulting in overkill, is the aim.

All these principles have been explored in great detail and at great length, with entire libraries filled with books on these matters. So it will do no good for any critic of mine coming here and saying, ‘Yeah, but what about this and what about that?’ I am fully aware of the limitations and the imperfections of these principles, and how most wars hardly are textbook examples of just warfare.

Take just one example. Many will object to the use of the atomic bomb in Japan. It certainly ended the war in Asia, and many have argued that the use of these two bombs ended up saving millions of lives. All that can be discussed and debated, and I have weighed in on it at various times.

Needless to say, just as societies change over time, so too does warfare. Modern warfare — be it total war, or nuclear war, or guerilla warfare, or terrorism, or weapons of mass destruction — raises new challenges and considerations. But the principles outlined here can still be of some use, even though they may well need to be tweaked and adjusted to better deal with modern realities and complexities. Many of the books I feature in my reading lists deal with these matters of recent developments in warfare and how to think ethically about them.

just war theoryFor further reading

Some four years ago, I put together a reading list on war and peace and related issues. It contains over 60 titles. While new books have come out since then, here is still a fairly up-to-date and useful list of books.

If I did have to add a few more titles, I would include these ten helpful and mostly recent volumes:

  • Brunstetter, Daniel and Cian O’Driscoll, eds., Just War Thinkers: From Cicero to the 21st Century. Routledge, 2017.
  • Coates, J. A, The Ethics of War, 2nd ed. Manchester University Press, 2016.
  • Corey David and J Daryl Charles, The Just War Tradition: An Introduction. Intercollegiate Studies Institute, 2012, 2018.
  • Dubik, James, Just War Reconsidered: Strategy, Ethics, and Theory. University Press of Kentucky, 2016, 2018.
  • Elshtain, Jean Bethke, Just War Theory. NYU Press, 1991.
  • Fotion, Nicholas, War and Ethics. Continuum, 2008.
  • Hall, Mark David and J. Daryl Charles, eds., America and the Just War Tradition: A History of U.S. Conflicts. University of Notre Dame Press, 2019.
  • Miller, Paul. Just War and Ordered Liberty. Cambridge University Press, 2021.
  • Orend, Brian, The Morality of War, 2nd ed. Broadview Press, 2006, 2013.
  • Paterson, Eric, Just American Wars: Ethical Dilemmas in U.S. Military History. Routledge, 2018.


Originally published at CultureWatch. Photo by Ivan Oboleninov.

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