Nine years after the terror attacks of September 11, 2001, we’ve all had plenty of time to think about war and its justifications. My own views have moved me steadily towards the viewpoint that war is hardly ever justified either morally or in terms of the rational self-interest of those involved. The obvious problem is that, if no one else is willing to fight, an aggressor could benefit by making demands backed by force. It seems to me, however, that this problem can be overcome by admitting that self-defense (including collective self-defense) is justified only to the extent of restoring the status quo ante bellum. That is, having defeated an aggressor, a country is not justified in seizing territory, unilaterally exacting reparations or imposing a new government on its opponent. Conversely, and regardless of the alleged starting point, countries not directly involved should never recognise a forcibly imposed transfer of territory or similar attempt to achieve advantages through war.
This isn’t a novel idea by any means, but I haven’t found an adequate discussion, and the discourse of International Relations theory seems to me worse than useless, being dominated by unrealities like ‘international realism’ , opposed to the strawman of ‘idealism’. Just war theory seems a bit more satisfactory, but I haven’t found it helpful in relation to the hard cases. 
Following up on the discussions we’ve had here recently, the status quo ante bellum position can be defended in game-theoretic terms, with no need to invoke unrealistic assumptions about international idealism.
The status quo ante bellum is, in game-theoretic terms, a salient point that provides a natural focus for a coalition, particularly for third countries with no direct stake. An interest in preserving international order provides third countries with a motive not to recognise gains secured by others through military force. While the gainer may be able to secure recognition from close allies, the absence of general recognition renders such gains costly and uncertain to hold, no matter how long they endure.
This isn’t merely a theoretical claim. International practice since 1945 has typically been to deny recognition to territorial gains secured by force, regardless of “facts on the ground”, no matter how long-lasting. Examples include the Indonesian annexation of East Timor (shamefully, Australia was one of the few countries to recognise this claim), the Turkish occupation of Northern Cyprus, the various ‘frozen conflict’ mini-states created under Russian patronage after 1991 and of course the Israeli occupation of the West Bank.
But there’s still, it seems to me, a widespread presumption that having gone to war in self-defense (as judged by themselves), countries are entitled to set whatever objectives they see as reasonable. The point of proposing status quo ante bellum as the only legitimate goal is to close off this capacity for self-defense to turn into aggression.
Of course, none of this would matter if the strategic use of military force reliably or even commonly yielded net benefits, as is assumed by ‘realists’ and, until relatively recently, by most holders of, and seekers after, political power. The experience of the 20th century shows the opposite – many cases of wars ending disastrously for those who launched them, and hardly any that produced clear and sustained net benefits.
I don’t intend merely the limited, and obvious point that, taken as a whole, the people of a country that goes to war are worse off as a result. Looking at the evidence, the same is typically true of the ruling classes, and of the individual rulers who make decisions to take their people to war. Of course, there are beneficiaries in the armaments industry and the military hierarchy, but almost any policy, no matter how disastrous, benefits someone.
I haven’t so far discussed the case for humanitarian intervention, punishment of war crimes and so on. The Iraq war showed that if individual governments (or coalitions) are allowed to set themselves up as judges in their own cases, disaster will ensue. The only legitimate basis for such action is a decision of the international community as a whole. At present, the best, admittedly imperfect, bodies for making such decisions are the UN Security Council and the International Criminal Court. The cumbersome nature of their processes mean that many wrongs will go unrighted, and many crimes unpunished. But that is true of all legal systems. And even if such interventions are justified, that does not mean they will be feasible or, if undertaken, assured of success.
Finally, most of what I’ve said about war applies to revolution and other forms of political violence. The fact that the existing order of things is unjust and not amenable to change is not sufficient justification for the certain suffering and far from certain benefits of revolutionary violence. Most revolutions, like most wars fail, and, as with wars, initial success is rarely enduring.
fn1. I’m aware that I’m an amateur dismissing the efforts of the professionals, and therefore likely to have got a lot of things wrong. But, as an economist, I’ve often been on the other end of this kind of criticism, and I think it’s useful to face it.
fn2. I mention this example because it would be silly to omit it, but I absolutely do not want to derail discussion into another rehash of this issue. Any comment referring in any way to the Israel/Palestine conflict will be deleted with prejudice.