Britain and the United States have been suffering from intervention fatigue. The reason is obvious: our interventions in Iraq and Afghanistan have proven far more costly and their results far more mixed and uncertain than we had hoped.

This fatigue manifested itself in almost exactly a year ago, when Britain’s Parliament refused to let the Government offer military support to the U.S. and France in threatening punitive strikes against Syria’s Assad regime for its use of chemical weapons. Since then, however, developments in Syria have shown that our choosing not to intervene doesn’t necessarily make the world a safer place. Nor does it mean that distant strife stays away from our shores.

There is reason to suppose that the West’s failure to intervene early in support of the 2011 rebellion against the repressive Assad regime left a vacuum for the jihadists to fill—jihadists whose ranks now include several hundred British citizens.

There’s also some reason to suppose that the West’s failure to support Georgia militarily against Russia in 2008, and to punish the Assad regime for its use of chemical weapons, has encouraged President Putin to risk at least covert military aggression in Ukraine. I’m not saying that the West should have supported Georgia and punished Assad. I’m merely pointing out that inaction has consequences, too, sometimes bad ones.

Now, however, despite out best efforts to keep out of direct involvement in Syria, we are being drawn in again. The rapid expansion of ‘Islamic State’, involving numerous mass atrocities, has put back on our national desk the question of whether we should intervene militarily to help stop them.

What guidance does the tradition of just war thinking give us in deliberating about military intervention? The first thing to say is that there are different streams in the tradition of just war thinking. In the stream that flows from Michael Walzer, the paradigm of a just war is national self-defence. More coherently, I think, the Christian stream, in which I swim, holds that the paradigm of a just war is the rescue of the innocent from grave injustice. This rescue can take either defensive or aggressive forms. The stipulation that the injustice must be ‘grave’ implies that some kinds of injustice should be borne rather than ended by war. This because war is a destructive and hazardous business, and so shouldn’t be ventured except for very strong reasons.

What qualifies as ‘grave’ injustice, then? In the 16th and 17th centuries just war theorists like Vitoria and Grotius proposed as candidates such inhumane social practices as cannibalism or human sacrifice. International law currently stipulates ‘genocide’. The doctrine of the Responsibility to Protest (‘R2P’) would broaden the law to encompass mass atrocity. Let’s suppose that mass atrocity characteristic of a ruling body is just cause for military intervention. Some nevertheless argue, in the light of Iraq and Afghanistan, that intervention is not an appropriate response, because it just ddoesn’twork. Against that conclusion, I call two witnesses, both of whom have served as soldiers, diplomats, and politicians, and have had direct experience of responsibility for nation-building: Paddy Ashdown and Rory Stewart.

Ashdown, the international High Representative for Bosnia and Herzegovina from 2002-6, argues that “[h]igh profile failures like Iraq should not … blind us to the fact that, overall, the success stories outnumber the failures by a wide margin”.

Rory Stewart was the Coalition Provisional Authority’s deputy governor of two provinces of southern Iraq from 2003-4. He approached the task of building a more stable, prosperous Iraq with optimism, but experience brought him disillusion. Nevertheless, Stewart writes that “it is possible to walk the tightrope between the horrors of over-intervention and non-intervention; that there is still a possibility of avoiding the horrors not only of Iraq but also of Rwanda; and that there is a way of approaching intervention that can be good for us and good for the country concerned”.

Notwithstanding that, one lesson from our interventions in Iraq and Afghanistan—and indeed from British imperial history—is that successful interventions in foreign places, which go beyond the immediate fending off of indiscriminate slaughter on a massive scale to attempting some kind of political reconstruction, cannot be done quickly or on the cheap.

Here’s where national interest comes in. National interest isn’t necessarily immoral. A national government has a moral duty to look after the well being of its own people and to advance its genuine interests. What’s more, some kind of national interest must be involved if military intervention is to attract popular support, without which intervention is hard, eventually impossible, to sustain. One such interest can be moral integrity. Nations usually care about more than just being safe and fat. Usually they want to believe that they are doing the right thing, and they will tolerate the costs of war—up to a point—in a just cause that looks set to succeed. I have yet to meet a Briton who is not proud of what British troops achieved in Sierra Leone in the year 2000, even though Britain had no material stake in the outcome of that country’s civil war.

It is not unreasonable for them to ask why their sons and daughters should be put in harm’s way.

However, the nation’s interest in its own moral integrity alone will probably not underwrite military intervention that incurs very heavy costs. So other interests—such as national security—are needed to stiffen popular support for a major intervention. It is not unreasonable for a national people to ask why they should bear the burdens of military intervention, especially in remote parts of the world.

It is not unreasonable for them to ask why their sons and daughters should be put in harm’s way. And the answer to those reasonable questions will have to present itself in terms of the nation’s own interests. This brings us back to Syria and Islamic State. Repressive though the Assad regime was and is, and nasty though the civil war is, it probably wasn’t sufficiently in Britain’s national interest to become deeply involved militarily in 2011. The expansion of Islamic State, however, engages our interest in national security more directly, partly because as part of the West we are its declared enemy and partly because some of our own citizens are fighting for it and might bring their jihad back onto our own streets.

We do have a stronger interest, therefore, in taking the risks and bearing the costs of military intervention to stop and to disable Islamic State, and of subsequent political intervention to help create sustainable polities in Syria and Iraq.