When we talk about things, says Peace Research Institute Oslo (PRIO) researcher Henrik Syse, we can talk about them normatively or descriptively.

When we talk about the normative (also known as the prescriptive) aspect of something, we're talking about something as it ought to be. Speaking descriptively, on the other hand, is more about describing how something is rather than how it could or should be.

Ethics, understandably, is a normative conversation.

Thomas Hobbes is a think often discussed when it comes to society and conflict, mostly because of his contribution to political science in the book The Leviathan. The Leviathan in question is an absolute monarch, and Hobbes thought we need one because human nature, without an authority to keep us in line, tends to fall back to a chaotic, violent state of nature. We need to be controlled if we don't want to live in chaos. And so, on the topic of war and international peace, he says that we can have domestic peace, because at the head of the country is a powerful Leviathan. But we can't have world peace because we don't have a big enough Leviathan, or at least not yet.

Immanuel Kant, coming after him, argued that peace is possible on the basis of norms that everyone would be counted on to respect. He criticizes Hobbes for living in a non-ethical world, one where ethics is only a set of rules that we can agree on (or be forced to accept). In this worldview, that there's nothing we can appeal to. But Kant looked to the medieval philosopher Thomas Aquinas (Dominican monk, long dead) in his description of a 'moral law', similar to the religious concept of natural law. It's something that can be appealed to beyond ourselves, which the American Declaration of Independence called inherent dignity, a force that is thought ot maintain its integrity even in the face of the Holocaust or atomic bombs.

But dignity can't be quantified, not really. And when we try to make abstract qualities like this a basis for international law and peace, it can get problematic. Some argue against the idea of international law, saying that while domestic laws are possible you can't enforce them on international affairs because they're all about territory, power and solving our differences by war. You can only influence other states using your will, power and guile. This line of thought is one aligned with something called political realism, which emphasizes the state and its capacities.

But skepticism about an international order goes further back than Hobbes: Dante, the poet of Inferno fame, describes the only way we can achieve it is through a one-world government.

There's the notion that ethics are subjective – more of a matter of taste. This is known as cultural relativism, and it's been a concept that allows for a lot of different values to co-exist alongside each other. But it also generates a familiar problem.

"What happens," Syse asks, "when you sit across the table from Goebbels? Will you really stand behind the idea that 'your truth is your truth and my truth is my truth' then?"

No international agreement about ethics exists, and its hard to come to that kind of agreement when there are so many interests (or ways of life) present around the table. But today we're talking about what international consensus (or ethics) might look like in the case of war.

When we talk about war there are two important, opposite approaches. Pacifism says that war is immoral and you can't agree to it because it's a moral compromise. Realism says that war has nothing to do with war: it's about power, interests and the interests of the powerful. While a realist policy would have no problem going to war (should the conditions be right), a pacifist country would let itself be invated and then make life a living hell for the occupiers.

Most of us, however, probably fall somewhere in the middle.

The earliest account in the Western world of pacifism is when Athens was fighting Sparta in the Peloponnesian war, recorded by Thucydides, and the island of Melos decided not to take a part. Athens asked politely to use its port for the war effort, with the alternative being slaughter for the men and abduction for the women and children.

Melos' denied the Athenians, saying their war was totally unethical. The Athenians, keeping their word, slaughtered Melos' men and abducted their women and children.

While ethics still might not trump international interests, in international affairs today it's still important to at least perform that you're taking ethics into account. Someone invades Iraq to protect democracy. Someone else annexes the Crimean peninsula to protect the Russian-speaking population. Even the most flagrant disregard to international law still attempts to cloak itself in moral concerns.

Then there's the argument for international order not from a sense of objective morality, but from the utilitarian realization that, really, things might be better for most of us if we have rules.

But can you really have rules for something as brutal as killing each other? This is precisely what the Geneva Conventions attempt to be. In an ideal world they wouldn't be necessary. But they're what we have.

It's hard to discuss the morality of nations – some may feel like they can be held to a standard of conscience, but other say that only leaders can be held accountable in this way.

The awarding of the Nobel Peace Prize to Barack Obama in 2009 was arguably an attempt to influence that presidency along the lines of peace, or as a way to communicate how the world was ready to hold America accountable. Obama, however, used his speech to justify acts of war. Others thought that it was a way to criticize George W. Bush's administration, particularly for the invasion of Iraq.

While Bush often gets criticized on moral grounds, Syse reminds us that he may have indeed felt a moral obligation to launch the Iraq war, and that his HIV and malaria campaigns in Africa were fairly successful. It's hard to make sweeping moral judgements once you get into the details.

But making moral judgements is what many of us feel compelled to do, and moral & political philosopher John Rawls tried to work through all these tricky bits by speaking to the ability to have common values by promoting what he calls overlapping consensus. To have this, it doesn't matter if you believe killing is wrong because God told you so or because your king told you so or because you think killing is worse than not killing. What matters is that there's at least a common consensus that killing is wrong and, no matter how we got there, we're there. So there's some kind of framework we can work with.

But there are still many who claim that everything behind our consensus, namely our beliefs and the process of reaching them, is still really important. By ignoring this process, the argument goes, we're not resolving the conflicts so much as putting them off indefinitely.

There's also the issue of self-interest vs. morality, with some people saying that enlightened self-interest can coincide with moral values quite nicely. Adam Smith is one person who believes that a better society comes from following our self-interest – being a better baker, for example, helps everyone get better bread.

But when we get into morality and ethics, there are three particular types that we can think about, each with a different focus.

An issue with liberal internationalism, though, is that if a state is weak or corrupt or whatever then those rights might not be protected. And generally the bigger and stronger countries end up deciding more and having more power internationally. Liberal cosmopolitanism calls for global citizenship, but it's a utopian idea right now and generates fears of a one-state power.

Despite all this, there is a way that people have talked about justice in international relations when it comes to war. The field is called just war theory. It has a long history both in the West and in the East, and it tries to respond both to political realism and to the pacifist by saying, respectively, yes, there are morals and how would you answer Hitler?

One of the first major formulations of just war theory was by Augustine of Hippo. He is considered (like Thomas Aquinas) a saint in the Catholic tradition, ane one of his main tasks was trying to sort out the consequences of the Roman Empire's Christianization in the fourth century. Before this, Christians were mostly apolitical and didn't have to think of war. Soldiers wrote to him asking for moral advice and whether they could do their jobs in good conscience. He was one of the primary Western philosophers to dwell on the question, a long line of minds going from Thucydides, Cicero, Augustine, Aquinas, Grotius, Kant and through to Walzer.

There are three major factors to think about when you think about just war:

All three aren't necessarily found in the same place. There's the case of Rommel, a leader in Hitler's army– he didn't have jus ad bellum, but he fought hard for jus in bello, even to the point of tearing up Hitler's orders to kill prisoners of war after interrogation. Then there's Arthur Harris, on the Allied side, who orchestrated the Dresden bombing and was one of the few top-ranking guys not to get a knighthood.

There are some points to think about when deciding whether you have jus ad bellum:

First you need to have just cause, meaning that you have to respond to aggression and not commit an act of unprovoked aggression. This can't be about revenge or resources. People often talk about self-defense when attacked, or when their allies are attacked, or to launch a humanitarian intervention or even 'defend values'. It can get controversial. Aquinas, the Dominican monk, made room for taking out your prince if he's particularly aggressive against his own people. There's also the question of a preemptive strike – if you see them about to push the missile button, that could be considered just cause. But how long ahead of time, if you suspect the attack, would you be justified in launching an attack yourself?

Then there's right intentions, which is about not starting a war in order to slaughter people. Or bombing them pieces. There should always be some kind of plan to restore peace.

Add to that the necessity for a legitimate authority, so that no one can declare war except for the person who is responsible for the public good. Meaning your leader (often a head of state). Some radical thinkers have issues with this one, and the question of legitimacy is under constant debate.

There's also proportionality where your response can't go beyond the initial act of aggression. For example, when Morocco took a Spanish island, Spain didn't opt to invoke all the might of NATO support. Connected to this is the question of whether or not a war (or a particular action) would lead to less death. Nagasaki and Hiroshima could be argued as having been way out of proportion, but others argue that they prevented more loss of life in the end.

Then one can talk about it being a last resort or not. This refers to the claim that we've exhausted all other rational means. An infamous example is Britain and France standing by when Germany took parts of Czechoslovakia – they thought there was still time to bargain with Hitler. History has not been kind to this particular case.

And the last one: likelihood of success. While a lot of people might claim American intervention in Syria matches most of the factors mentioned above, there's less confidence about this one. You're expected to have an exit strategy, not to getting mired in the trenches. The war is supposed to be over by Christmas. Norway and Denmark were attacked by Germany on the same day and, while Denmark gave in after two hours, Norway fought on. They couldn't win, but the argument could be made for moral success being a sign of success – a sign of resistance was all that could be expected and was achieved. The king and parliament had time to escape. They gave the Nazis their very first military defeat. Which then lasted for two days – after that they were crushed by German troops. They're proud of this to this day. Syse is Norwegian and we wonder about his romanticism.

When talking about jus in bello, there are the twin principles of necessity and precaution – is this action really needed, and are you careful to hit the right target? Then there's proportionality again, not using weapons that will cause undue loss of life like chemical weapons or nuclear bombs or anything that's malo in se wrong in itself). Then there's distinction/discrimination, which is about avoiding civilians and other non-combatants.

One also needs to be aware that sometimes there's a double-effect and that maybe you'll have unintended consequences.

But there will always be dilemmas. There's the story of when the Allies in WWII discovered that the lab where the Germans were building their nuclear bomb was going to be moved. The only time the Allies could get a clear shot was during a short ferry ride over a lake. The problem was that there were something like seventeen civilians inside. The civilians were not their target, and killing civilians would be immoral, but they were also thinking that, well, seventeen dead now is better than tens of thousands dead in a nuclear blast. Make of that what you will.

"If you agree to that," Syse continues, "would you feel different if there were fife hundred people and a mere dozen or two?" But he doesn't like focusing on the extreme examples in order to set moral norms – you could justify doing nearly anything that way. A famous quote goes "the fact of twilight does not eliminate the difference between night and day." The allies decided to sink the boat.

We did not have time to talk too much about the principles of jus post bellum, but Syse lists them quickly:

"But obviously," he concludes, "it is easier all around if one does not go around starting wars."