This month, Saudi-led coalition forces recommenced airstrikes on Sanaa, the capital of Yemen, targeting the forces of Ansar Allah (known as the Houthis), after peace talks that began back in April broke down. The coalition – which includes forces from Bahrain, Egypt, Jordan, Kuwait, Morocco, Qatar, Sudan and the United Arab Emirates – supports ousted President Abdu Rabu Mansour Hadi of Yemen, whose government is still recognized by most countries, and has been at war with the Houthis since March 2015.

With the recommencement of hostilities, allegations also immediately resumed that the coalition forces were again targeting civilians and civilian structures. It is alleged that coalition airstrikes this month struck a potato chip factory, a school, a Médicins Sans Frontières-supported hospital, and a bridge used to bring 90-percent of World Food Program food to the capital. Similar charges have been made against the coalition since the beginning of the conflict, which has resulted in over 3700 civilian deaths.

Do these coalition strikes constitute war crimes? The law is relatively clear, if not exactly precise. It is a war crime to directly target civilians or civilian structures or to use force that results in civilian casualties disproportionate to the expected military objective. See para. 190 of the International Criminal Tribunal for the former Yugoslavia’s Galić Appeals Judgment: 

According to the principle of distinction, warring parties must at all times distinguish between the civilian population and combatants, between civilian and military objectives, and accordingly direct attacks only against military objectives. These principles establish an absolute prohibition on the targeting of civilians in customary international law but do not exclude the possibility of legitimate civilian casualties incidental to the conduct of military operations. However, those casualties must not be disproportionate to the concrete and direct military advantage anticipated before the attack (the principle of proportionality).

An attack that is indiscriminate – that fails to distinguish between military and civilian targets – can also constitute an unlawful attack on civilians: “[A]ttacks which employ certain means of combat which cannot discriminate between civilians and civilian objects and military objectives are tantamount to direct targeting of civilians.” (Galić, para. 132). With respect to the principle of criminal intent, or mens rea, the crime is committed under customary international law if civilians are targeted intentionally or recklessly (the International Criminal Court [ICC] Statute limits the crime to intentional attacks. See also, Galić at para. 140). Therefore it is not sufficient for the attacking forces to take just some precautions before striking; the precautions must be sufficient to eliminate any awareness of a substantial risk that civilians will be directly targeted or disproportionately killed or injured. In other words, forces within the Saudi-led coalition may be guilty of war crimes even if they did not deliberately target civilians or use disproportionate force, but instead did so recklessly (e.g., with awareness that their targeting practices would unnecessarily kill many civilians).

So much for the law. It’s when we reach the facts that things become difficult. First, given the chaotic circumstances of armed conflict and airstrikes (which by their nature arrive unexpectedly), it can be very difficult to establish what exactly happened on the ground: what was struck, by whom, by what kind of weapon, and the results of the strike. Access to evidence (such as the scene itself or to witnesses) may be delayed, partial or fragmented, and different investigators may or may not be affected by their own biases or perspectives. And the determination of lawfulness can turn on very fine-grained determinations. It is not enough to show that civilians were killed; it must be established that there was no legitimate military objective (the determination of which can be itself a contentious question), or that the civilian deaths were out of proportion to that objective.

To further complicate matters, it is not enough to figure out what happened on the ground. To determine if a crime was committed, it must also be known what those responsible for the strike knew at the time of the strike. If attacking forces take all reasonable precautions and believe that, on the basis of ordinarily reliable intelligence methods, a target is a military one, but it turns out they are wrong, that will constitute a terrible tragedy, but not a crime. Establishing what was known by the attacking forces at the time of a strike requires access to those responsible for the targeting decisions, the intelligence available to them at the time, and their communications.

These evidentiary challenges explain why these cases are often so disputed, and why there have been relatively few prosecutions for conduct of hostilities crimes at the international tribunals. Proving criminality ordinarily requires either unambiguous proof of an attack on a civilian target (as when Milan Martić directed the indiscriminate firing of cluster munitions on Zagreb in May, 1995, and then essentially bragged about it afterwards), or a clear pattern of attacks demonstrating an intent to target civilians (as in the siege of Sarajevo prosecutions of Generals Galić and Milošević).

The factual challenges also explain why there will also disputes about who conducts the investigation. The party with the best access to the information about what the attacking forces knew at the time of the attack is also, of course, the party that has the greatest interest in vindicating the attacks. On the other hand, military forces will often resist allowing outside investigators to have access to their internal communications and intelligence.

Regarding the coalition attacks in Yemen, all of these factors are in play. Disputes have emerged about what happened with certain specific strikes. The coalition created an independent Joint Incidents Assessment Team (JIAT) that recently released its findings regarding eight strikes in 2015 and 2016 (without explaining why these eight incidents were selected out of the many allegations). The JIAT disputes many of the factual allegations advanced by the UN or Human Rights Watch regarding these incidents, and asserts in several cases that the strikes were justified by “intelligence” available to the coalition forces. But at the same time it has not made public any of the evidence that it relied on, and therefore it is impossible to assess the value of the investigation. Moreover, the multiplicity of allegations of airstrikes on hospitals, mosques, marketplaces, restaurants, universities, and houses suggests a pattern evidencing, at a minimum, recklessness in targeting, but even establishing a pattern requires a detailed examination of individual incidents, which to date has not occurred.

In the end there are strong reasons to be concerned that war crimes have occurred, but greater certainty will require a more thorough, transparent investigation. It is no surprise then that today the UN High Commissioner for Human Rights, Zeid Ra’ad Al Hussein, called for an independent, international investigation.