Air strike in Sana’a, May 2015. Image by Ibrahem Qasim – Wikimedia 

If the United States and United Kingdom (have) become not just supporters of the Saudi-led coalition in Yemen but instead “co-belligerents,” is there a special duty on the US and British governments to investigate potential war crimes by the Saudis and their partners? This question is even more salient today given concerns raised about the quality of the Saudi coalition’s own investigations clearing itself of wrongdoing. It is also more relevant than ever given recent US military bombing of Houthi targets in Yemen.

The Saudi-led coalition is caught in an armed conflict with the Houthi rebel groups that have overthrown the internationally recognized government of Yemen. While the US and UK provide arms and other support to the Saudis, their officials have been at pains to stress that their governments are not party to the Saudi-coalition’s armed conflict with the Houthis.

That line has become increasingly hard to draw with the recent exchanges of fire between the US and Houthi forces. It was a hard line to maintain in the first place. For an independent legal analysis, see Just Security’s Nathalie Weizmann’s post applying the legal test of co-belligerency to ongoing US and UK assistance (including arms sales and refueling) to the Saudi-coalition.

Becoming a party to an armed conflict has many implications. One of those may include the duty to investigate fully and independently alleged war crimes by your allies—and not just to defer to them in the process. So what might those duties entail?

In a Written Ministerial Statement, a senior U.K. official appeared to acknowledge that his government’s duties to investigate would be different if the UK were a party to the conflict. He wrote:

“It is important to make clear that neither the MOD [Ministry of Defense] nor the FCO [Foreign & Commonwealth Office] reaches a conclusion as to whether or not an IHL [International Humanitarian Law] violation has taken place in relation to each and every incident of potential concern that comes to its attention. This would simply not be possible in conflicts to which the UK is not a party, as is the case in Yemen.”

What about the other side of the pond? For what it may be worth, a Reuters story reports:

“U.S. government lawyers ultimately did not reach a conclusion on whether U.S. support for the campaign would make the United States a “co-belligerent” in the war under international law, four current and former officials said. That finding would have obligated Washington to investigate allegations of war crimes in Yemen …”

Can that be right? What does the law actually say here? Is there a special legal obligation to investigate alleged war crimes if the US (or UK) is deemed a party (or “co-belligerent”) to the conflict? I have asked legal experts with far better understanding than me to answer that question. Watch this space for coverage of this issue at Just Security.