The terrible events in Iraq have revived calls for intervention even among those who opposed the 2003 war, and even among some with anti-interventionist sentiments generally. Here I simply wish to lay down, as briefly as a post allows, the status questions in the academic literature on humanitarian intervention and tentatively suggest how it may apply to the events in Iraq. I will not address issues of policy, statecraft, strategy, domestic law and politics, and the like (although these may turn out to be the most important.)

The international law of intervention

Because of the centrality of state sovereignty, military intervention is in principle banned by the UN Charter (art. 2 (4)). Yet the predominant view is that humanitarian intervention can sometimes be lawful, subject to several requirements:

1) The intervention must be aimed at stopping or preventing large losses of life.
2) The intervention must be proportionate, like any other use of military force.
3) The intervention must be authorized by the UN Security Council. This has became the prevailing view after 2005, when the United Nations Summit adopted the Responsibilty-to-Protect standard. Unauthorized intervention is unlawful (but there are dissenters.)
4) The right of the intervener to use force ends when the threat of loss of lives is removed.
5) Sometimes the intervention may require regime change (when the government is the perpetrator.) In these cases, the preferred action is to hand the country to the UN or some such body. If the intervention does not require regime change, then the intervener must leave after completing its mission.
6) The intervener must observe the laws of war (Geneva conventions.) Notice that these laws do not prohibit collateral damage (the killing of innocent civilians). They merely require that such damage be incidental to the intended destruction of a legitimate military target.

Under international law, air strikes are as much use of military force as ground invasions. So the current air strikes in Iraq, had they been undertaken without the consent of the Iraqi Government, would have been unlawful unless in conformity with 1-6 above.

But here, aside from the compliance with proportionality and the laws of war, there are no international law issues because the foreign bombers are there by consent of the territorial sovereign, the Iraqi Government. So from the standpoint of international law, this is not technically an intervention: it is a request for help by a sovereign government who is being attacked by a rebel group. International law allows such request. I should add that IS’s truculent crimes reinforce the legal position of the Iraqi Government and its current Western allies. Recent resolutions by the UN Security Council sympathize with the air strikes, but this is because the Iraqi Government has authorized them. So I believe that the current intervention in Iraq is lawful, as would be a ground invasion. The intervener needs no UN Security Council authorization when it is invited by the territorial sovereign.

The ethics of intervention

Philosophers are generally more favorable to humanitarian intervention than lawyers (although dissenting voices abound.) Here are the conditions of legitimacy usually discussed:

a) The intervention must be aimed at saving persons from massive attacks by their governments or others. It is a case of defense of persons who are victims of unjustified attack, defense of others. But philosophers tend to be  a bit more flexible about this threshold. Some would accept humanitarian intervention in the case of severe tyranny or anarchy (which need not reach genocidal proportions.) Others would even accept intervention to restore democracy —at least if the cost to the intervener and to innocents is low.
b) The requirement of proportionality holds, but in ethics it reaches further than in law. In the law of intervention, proportionality is limited essentially to the battlefield and to harm to noncombatants. In contrast, an ethical analysis must consider all  the consequences, in particular the possibility that intervention may make things worse in the long run.
c) Collateral damage in a humanitarian intervention is governed by the strictures of the doctrine of double effect, although there are influential voices that deny the relevance of the doctrine and would ban all interventions, or at least those that were not strictly surgical.
d) If humanitarian intervention is substantively justified, then the intervener does not need the authorization of the Security Council or anyone else (this is a major difference with the legal position.)
e) Unlike international law, who couldn’t care less about how the state recruits and finances its armed forces, the morality of intervention, under the influence of libertarian ideas, addresses the internal justification of intervention, that is, the question under what circumstance, if any, a government may tax its citizens to finance a humanitarian intervention. Many who accept the legality of humanitarian intervention in a particular case deny the power of government to force citizens to pay for it. This leaves untouched, of course, the use of privately-financed mercenaries or voluntary brigades.
f) The literature usually adds the requirement that the intervention must be welcome by the victims.

Significantly, the notion of sovereignty has a limited role in the ethics of intervention. This is because many philosophers think that state sovereignty has a moral purpose that does not include the state’s massacring its own citizens. But there are dissenting voices here as well.

As I indicated, I think a military intervention in Iraq to stop IS would be lawful under international law because it would not be, legally, an intervention at all. From an ethical standpoint, given the horrific crimes of IS and the threats it poses, I think the intervention would be legitimate, provided the internal and external costs, including long-term costs, are morally acceptable. And this, of course, is the crucial question.

These are the bare bones. There are of course many more issues, which Bas van der Vossen and I plan to address in a book for Oxford University Press entitled Debating Humanitarian Intervention, where we hope to be much more thorough (and with Bas’s help, more accurate) than I have been here.

Postscriptum: Humanitarian interventions, both in law and morality, are merely permissible, not mandatory (unless, perhaps, in ethics, when the cost is near zero.) So nothing I wrote speaks to the wisdom of intervening in Iraq (especially since the cost will not be zero), assuming the relevant conditions are met. Nor do I necessarily endorse all these rules and conditions. My aim was merely to summarize the literature.