Just War Theory and the Ethics of Armed ConflictJust War Theory and the Ethics of Armed Conflict

Just War Theory and the Ethics of Armed Conflict

Humans have waged wars for millennia, but is war ever justified? Theorists provide some answers.


September 1, 1939. Hitler launches a blitzkrieg against Poland. From sea and air, Germany bombs everything, including strategic targets like forts, munition dumps, and railroad lines, to civilians in passenger trains, buildings, and on the streets. So begins World War II.


Hitler claimed Germany’s action was justified. Regaining territory lost in the armistice agreement ending World War I and responding to Poland’s alleged persecution of Volksdeutschen – ethnic Germans – were the ostensible reasons for Germany’s unprovoked attack. (If this sounds rather like Vladimir Putin’s rationalization for invading Ukraine, well, it does. More on that later.) Even if true, would those reasons warrant attacking another country, with or without an official declaration of war?

German planes fly over Poland. (Source: U.S. Department of Defense)

German planes fly over Poland. (Source: U.S. Department of Defense)


August 6, 1945. U.S. President Harry Truman orders a single bomb to be dropped on Hiroshima, Japan. Dubbed “Little Boy,” the more than 9,000-pound atom bomb immediately annihilates approximately 80,000 people. Tens of thousands more die in the aftermath, and the long-term effects are still imprecisely understood. 


Some claim that Truman’s decision saved countless more lives than were wiped out in Hiroshima (and, three days later, in Nagasaki). The actions, they conclude, were justified. But were they?


To think about this question, let’s look at just war theory and the criteria that thinkers use to assess the justifications for military conflict.


For more on the human cost of war, watch on The Blitz: Britain on Fire MagellanTV.


Moral Use of Force Depends on Whom You Ask

Whether on the playground or in the global arena, bullies are roundly condemned. Part of the reason may be our general sense that might just doesn’t make right. If you want the power to do what you want when you want, without having to answer to anyone or anything, you think rules don’t apply to you – except the rule that says doing whatever it takes to get power is acceptable. Getting what you want is the first and last rule of conduct. Scaled to countries, this attitude is the basis of the delusion of grandeur in which people like Hitler lived.


Responding to an unprovoked attack – self-defense – is arguably the most justifiable reason to go to war. 


Of course, you might genuinely believe that what you think is good for you is also good for others – you may not be entirely self-serving. Even so, it’s not a sufficient justification to encroach on another’s autonomy or sovereignty.


Nevertheless, it’s thinking like this that led to the expansion of numerous empires from ancient times through the 20th century. For example, when the early Christian church spread across the Roman Empire, and later throughout Europe, no one was worried about whether or not subduing and colonizing pagans was a massive overreach. No, these worshippers of the Lamb of God were convinced of the righteousness of their cause.


But are there more persuasive arguments to be made in a rational consideration of the decision to go to war? People who have thought long and hard about the subject have a framework for such a grave decision: They call it just war theory.


Is War Ever Justified?

Just war theory lays out the criteria for declaring, waging, and dealing with the aftermath of war. Such theories generally focus on four main classifications:


  1.  Jus ad bellum, the Latin for “justice of war”; 
  2. Jus in bello, which translates to “justice in war”;
  3. Jus ex bello, or jus terminatio, for “justice in termination”; and
  4. Jus post bellum, or “justice after war.” 


Let’s focus our attention on the first two. Is war always, sometimes, or never wrong? Most of us would say that war is sometimes wrong or, contrarily, that war is sometimes right. You might be a pacifist, however, and declare that war is always wrong. Alternatively, you might take the view that wars are never wrong, at least insofar as you believe war is an inevitable feature of survival.


February 24, 2022. Russian president Vladimir Putin launches an all-out invasion of Ukraine. The justifications for the so-called “special military operation” include the “demilitarization and denazification” of Ukraine’s government, and the “liberation” of Ukraine’s Donbas region. Moreover, in Putin’s view, Ukraine and Russia are, historically, “one people.” On this view, consequently, Ukraine does not have a legitimate claim to sovereignty.


Putin also insisted that the West – specifically, the United States and other member countries in NATO – threatened Russia’s very existence. In order to ensure its security, Putin asserted, Ukraine must be neutral. Neutrality, in turn, had to be enforced by violence. 


More than 100 million people were killed in wars in the 20th century alone.


The international response was swift and decisive. NATO, for example, condemned the invasion. It declared Ukraine “an independent, peaceful and democratic country” that had done nothing to provoke Russia’s attack, calling it “unjustified.” And, backing up its words, the alliance opened up a pipeline of lethal weapons to Ukraine. So, who’s right – or are both wrong?


The Justifications for War

Theorists generally agree on six principles for justifying war: 


  1. The cause must be just: Going to war aims to avoid (greater) harm.
  2. The intention must be right: The cause of war is the goal, rather than a pretext for some other goal.
  3. The authority must be legitimate: The war is fought by the entity that has the proper authority to declare and wage war.
  4. There is a reasonable prospect of success: The war is likely to achieve its aim.
  5. There is proportionality of goods and bads: The moral goods of the war outweigh the moral bads.
  6. War is a last resort: The war is necessary; there are no other options for achieving the just cause.


Presumably, all six criteria ought to be met before waging war. That said, working out the details may quickly lead one into the moral weeds. Recall, for example, the discussion of people who genuinely believe they must fight the good fight. Let’s suppose that Vladimir Putin truly believes that Russia would eventually be annihilated if he hadn’t ordered the pre-emptive attack on Ukraine. Going down the list above, it’s not hard to see that he would also believe that Russia would be successful, therefore that the ultimate benefit to Russia outweighs the costs to the Ukrainians. 


U.S. soldiers arrive in Germany in support of NATO allies, 2022. (Source: U.S. Department of Defense)

U.S. soldiers arrive in Germany in support of NATO allies, 2022. (Source: U.S. Department of Defense)


Notice also that Putin could claim the other criteria are satisfied. If we disagree, however, Putin simply ticking off each item in the list isn’t sufficient for war. There are judgments to be made about what counts as a good reason in the matter at hand. 


For all its sincerity, a belief may be wrong. In other words, thinking doesn’t make it so, anymore than might makes right. Even a biased but rational observer could see that Hitler and Putin did not do everything in their power to prevent war. Then again, when they rise to power, megalomaniacs cut the bonds of justification, thereby untethering action from reason.


Does Self-Defense Always Justify War?

Megalomaniacs or not, leaders make errors. It is now generally agreed that the U.S. invasion of Iraq was a terrible mistake, not adequately justified by at least half – let alone all – of the principles that justify war. For example, what began as self-defense – a search for Osama bin Laden in Afghanistan – morphed into the U.S. argument, presented at the United Nations in 2003, that Saddam Hussein had biological weapons.


Even if not the exclusive reason for invading Iraq, that argument was a significant justification. A protracted war nevertheless followed after and, along with it, grossly unacceptable conduct, such as the torture and abuse of prisoners in Abu Ghraib.


On the other hand, self-defense, as when one nation assaults another’s territory, is arguably the most obvious just cause. Assisting another country against an oppressive regime or invasion may also be just causes of war. If that’s the case, however, we might wonder why there isn’t a moral imperative to act every time a violation occurs. 


For example, the United States and other countries did not directly intervene when Syrian President Bashar al-Assad violently crushed pro-democratic demonstrations, killing thousands of his fellow citizens. Why not? Why didn’t the United States and other countries intervene in the genocide of the Tutsis by the Hutus in 1994, during the civil war in Rwanda? Why didn’t the United States enter World War II before Japan attacked Pearl Harbor in December 1941, more than two years after Hitler invaded Poland, and long after it was clear that Hitler was planning mass genocide?


These questions reveal the distinction among perceived or actual national self-interest, morality, and the activity of war. While morality may demand intervention, for example, and while using military might to aid others may be justified, political interests may discourage action, as happened in the U.S. during the first years of World War II. Prior to the attack on Pearl Harbor, substantial numbers of citizens and their Congressional representatives were dead-set against involvement in the war; it took that disaster to overcome their opposition.


The Conduct of a Just War

Let’s grant that war is at least sometimes justified. What can be said about the conduct of war? In other words, once a war breaks out, are there specific moral rules governing its conduct? 


Just war theorists recognize six basic rules of conduct, broken out from three principles found in international law:


  1. A justly conducted war obeys international laws on prohibited weapons (e.g., biological and chemical).
  2. Soldiers in a justly conducted war use their sanctioned weapons against legitimate targets (those involved in the rights-violating harm), which means they must never target civilians.
  3. Soldiers in a justly conducted war use only proportionate force, i.e., the force required to achieve specified military ends.
  4. POWs must be treated humanely in a just war. The Geneva Conventions articulate “benevolent” quarantine.
  5. Soldiers in a just war must never use methods or weapons that are mala in se (evil in themselves). Mass rape and ethnic cleansing are examples of prohibited methods, and biological weapons are an example of prohibited weapons. 
  6. States involved in a just war must not seek reprisals that violate any requirement of conduct in a just war. When one state violates one or more requirements, and another state seeks reprisals in kind, both sides violate just war conduct.


By its very nature, war is violent, destructive, and deadly. Informal and formal rules of war existed, nevertheless, as far back as civilizations like the ancient Greeks, Babylonians, Israelites, and Chinese. But why? Why respect boundaries if physical violence is the means to the stated end? It sounds vaguely reminiscent of the inconsistency in the expression, “honor amongst thieves.” American revolutionaries used guerilla tactics against the British to notably effective ends, hardly concerned about whether or not the British would say such conduct was “not on.” 


F4F-4 Wildcat fighter taking off from the USS Yorktown, Battle of Midway, June 4, 1942. (Source: U.S. Department of Defense)

F4F-4 Wildcat fighter taking off from the USS Yorktown, Battle of Midway, June 4, 1942. (Source: U.S. Department of Defense)


You might object that we agree to rules in order to protect people not directly involved in war. The point of war is to quash the enemy. If the titular head of the enemy is the government, it would seem that military decapitation is the means to vanquish that enemy, not annihilate non-combatants. But does doing so merely push the question back? Out-and-out chaos, “scorched earth,” or “shock and awe” – the sort of overwhelming violence that obviates any response – would seem to be the most effective approach. And that would necessitate civilian casualties. 


In war, don’t we revert to a sort of pre-societal state of nature, such as the one described in Thomas Hobbes’ Leviathan? In his thought experiment, Hobbes claimed that the state of nature is a ‘war of all against all,’ a condition in which individuals are totally free and equal, but also without laws. Peace is achieved by entering into a social contract that gives power to a sovereign, such as a king or a duly constituted democratic government.


Individuals want security and peace. So do nations. Hence, the sorts of international agreements that are codified into law or that form alliances. Some agreements detail justifications for going to, and conducting war. Which brings us back to where we began when we asked why anyone would agree to rules guiding the conduct of war. And while we’re at it, we should revisit the rule against civilian targets. If nothing else, Hitler’s decision to bomb civilian targets in Poland, Truman’s decision to drop atomic bombs on Japan, and Putin’s decision to bomb schools, apartments, and maternity wards should be sufficient evidence that some wartime activity is not so very distinct from ordinary criminal conduct.


The Future of Just Wars

Wars will doubtless plague us as long as we humans run the show on planet Earth. For all our advancements in the sciences, for all our beautiful works of art, and for all our lofty ideals, many of our fellow human beings remain violent and predatory creatures. There will likely always be a version of the dictator, just as there will likely always be a version of the serial killer, the domestic abuser, and the mass murderer. Nations will continue to come to other countries’ aid – or plead for help themselves – just as they will often ignore injustices elsewhere and at other times. Would the U.S. and NATO have helped Ukraine if it weren’t in Europe and populated by folks who “look like us”? 


NATO’s geographical proximity to Russia raises the specter of danger for member countries, and Europeans generally, in a way that a distant conflict on another continent would not. So, we can’t fault the response where that is the motivation. On the other hand, if we believe unprovoked attacks are unjustified and unacceptable wherever they occur, then something has gone morally awry when countries or alliances like NATO are silent.




Mia Wood is a professor of philosophy at Pierce College in Woodland Hills, California, and an adjunct instructor at various institutions in Massachusetts and Rhode Island. She is also a freelance writer interested in the intersection of philosophy and everything else. She lives in Little Compton, Rhode Island.


Title image: Residential building in Borodianka settlement (Kyiv Oblast of Ukraine) on Tsentralna Street, 359, after Russian bombing in the morning of March 2nd, 2022 via Wikimedia.


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