When thousands of people gathered in cities across Britain in November 2015 to protest against proposals to bomb Daesh in Syria, many expressed exasperation at the government’s failure to learn from the wars in Afghanistan and Iraq. Neither bombers and missiles nor soldiers on the ground had succeeded in bringing peace and justice to those states when they were deployed soon after 9/11. Nor did they succeed later when used to support anti-Gaddafi rebels in Libya in 2011. The protesters of 2015 therefore believed that those millions of their predecessors who had turned out in 2002 and 2003 to oppose Tony Blair and George W. Bush had been proven right.

But what exactly was the message that government ministers should have been listening to? And what was the lesson they should have learnt? I myself was part of those earlier protests. By 2002, I was increasingly worried by what seemed to me to be a dangerously misguided foreign policy. The impending Iraq intervention didn’t seem justified by a well-founded fear of attack. More especially, I thought that invading another country with a view to changing its regime from an authoritarian one to a constitutional democracy was wrongheaded. Even though the intended outcome was a worthy one, as I was prepared to accept, it seemed to be based on a deficient understanding of the nature of political legitimacy: without clearer evidence of a will on the parts of Iraq’s multiple peoples to ‘brave labour and danger for their liberation’, to use J. S. Mill’s words, not to mention a well-thought-out plan for securing it, the project was doomed to cause a violent breakdown in such order as Iraqis enjoyed (Mill 1984 [1859]: 122). These were the reasons why I joined the demonstrations in Dublin, where I lived at that time.

But I was also frustrated at the message expressed by some of my fellow protesters – a message that many who protest nowadays continue to carry forward. They weren’t only marching against this particular war. Their protest was against war in general. By contrast, I wasn’t convinced that pacifism offered the best starting point for debating a case like the Iraq invasion. Pacifists have a steep hill to climb in order to persuade others that war in any particular case is unjustified. To succeed, first they have to persuade people of something that’s very hard to prove: that war can never be justified. That ‘never’ might literally mean ever: neither in the present circumstances of world politics, nor during any other era of human history. Or it might be limited to the era of total war, of nuclear arms, of ‘new wars’, or of whatever it might be: ‘never’ might mean ‘in none of the cases we are likely to face in the foreseeable future’. But either way, I believe that most of the people that protesters seek to engage will be convinced to begin with that war is justifiable in at least some imaginable cases, however peculiar or extreme. In particular, they are likely to believe that a world that continues to resemble in any essential respects the world of the 1920s and 1930s cannot rule out a priori the possibility of a just war.

What I will call ‘the just war idea’ – the belief that war is sometimes justifiable, provided it is initiated in the right circumstances and conducted and concluded in the right way – is, as many might think nowadays, part of our global ‘common sense’. It contributes to what Michael Walzer called ‘the ordinary language in which we argue about particular wars, whether we speak as political representatives, journalists, academics, or simply as citizens’ (2004: x). But the idea has been criticized. Not only do pacifists raise doubts about it, but so too do some who argue from a sharply contrasting position: realists in foreign policy sometimes question the plausibility of trying to apply moral principles in war and contemporary international politics more generally.

Some criticize the just war idea in another way. They maintain that just war theory helps license aggression and neo-colonial adventurism by offering the leaders of powerful states a ready-made rhetoric of public justification.¹ This may be true, but the position for which I will argue is that just war theory is also an indispensable tool for criticizing those who promote war. As long as world politics holds up the possibility – however infrequent – that war may occasionally be morally necessary, then we will continue to need just war theory to guide debate and action in contemporary politics. That way, when some state or other organization makes a bogus claim to have justification for resorting to arms, as will inevitably and all too commonly be the case, the best critical reply will be to argue, not that war is never justifiable, but that this war is unjustified because it fails by the standards of the just war idea.

Instead of arguing from a pacifist position, then, it seems to me that the best way to debate the justifications offered for war, first in Iraq, and then in the various other cases over the years since 2003, was on the basis of what will be common ground for many people: the belief that, as it were, ‘just war’ is possible, at least in principle. Each particular case ought to be debated in light of that possibility: is it a case that satisfies the necessary criteria of a just war? Or is it not? In this way, we can debate wars in prospect, as when people asked in 2015 and 2018 should the United Kingdom join the United States in bombing Syria (or, as it might be, in attacking North Korea or Iran)? We can debate the justification and the conduct of a war as it unfolds. And then we can debate it in retrospect.

To make the case that just war is possible, I argue that we need to rethink the terms in which the very idea of a just war is understood. First, we need to recognize that just war theory is as much about justifying war as it is about restricting it and restraining those who participate in it. Second, I think we need to recognize greater affinities between just war theory and contributions to political ethics that are more often identified with ‘realism’. The vitally important moral limits on human action that just war theory is built around, particularly in its contemporary, liberal form, have to be seen as part of a wider theory that seeks to achieve the best possible balance between moral restraint and moral necessity. For those in positions of political leadership – whether as presidents and prime ministers or as revolutionaries and rebels – these may be characterized using Max Weber’s terms by way of a contrast between moral conviction and political responsibility. But I also want to emphasize the affinity between just war theory at its most basic level of commitment and some varieties of what’s known as ‘contingent’ pacifism.

In chapter 1, I sketch the historical origins of the just war idea and review some lines of division between different contemporary schools of just war thinking. I then turn in chapter 2 to a key question: what else do you have to believe in order to believe that just war is possible? Answering it clarifies just what it is that we need to defend if we are to provide sufficient grounds for believing that just war is possible. I argue that the just war idea relies on a set of basic convictions and assumptions that are both prima facie plausible and likely to be shared even by many people who wouldn’t think of themselves as followers of just war theory.

I then respond to a series of possible objections to these beliefs and, hence, to the possibility of just war. Chapter 3 focuses on problems with what just war theorists nowadays call the jus ad bellum (that part of the theory that concerns the justice of going to war), and chapter 4, on the idea of a jus in bello (the theory of right conduct in war) and the compromises that might be necessary in concluding otherwise just wars (questions of jus post bellum). In the course of my defence, I build up an interpretation of the just war idea that resists both overly formal and excessively idealistic approaches to the question. My claim is ultimately that just war is possible if it means war that is all-things-considered justifiable in the circumstances. This requires taking account of the inevitability of error, compromise and human failure. A possible just war is ultimately one that is less evil than the evil it resists, even if this typically means falling some way short of the ‘ideal’ image of just war envisaged in theory.