There has been surprisingly little discussion about the international legal justification for the airstrikes in Yemen that are being led by the Saudis. Other than a Just Security post that tackles some of the legal issues, the media and those using force have spent almost no time discussing whether the Saudi-led coalition's intervention raises legal questions. That in itself is notable. States seem to be making fewer efforts these days to discuss their international justifications for using force (with a few exceptions), and few other states are demanding clear explanations for the force. (Consider, for example, the airstrikes by Egypt and the UAE in Libya in August 2014, which both states refused to discuss.) This could become an even more salient problem if the regional military force that the Arab League is discussing takes shape. The existence of such a force increases the likelihood that we will see military interventions by states that do not seem committed to articulating legal justifications.
The coalition's justification is that they have the consent of Yemeni President Abed Rabbo Mansour Hadi. Hadi himself wrote a letter to the Security Council in which he asked the Security Council to authorize a military intervention to "deter Houthi aggression" and stated that he had asked members of the Gulf Cooperation Council and the Arab League to intervene militarily. He also invoked Article 51 of the Charter. This is odd, because Article 51 would only be relevant if Yemen (or the Saudi coalition) were asserting that Yemen was responding to an external armed attack. The Houthis are a rebel group whose roots are internal to Yemen. So unless Hadi intended to signal that he sees the Houthis as a front for Iran and plans to treat Iran as the true author of the Houthis’ violence, this invocation of Article 51 is misplaced. (And if he and the Saudi coalition believe that they are acting under individual and collective self-defense against Iran, they should file Article 51 letters. I have previously written about the increasing lack of such filings here.) More cynically, claiming self-defense to respond to “external” problems is a way to divert blame away from a regime’s own internal failures.
Assuming that the Saudi coalition is acting on Hadi's consent to avoid any Article 2(4) problems, we might wonder about the strength of that consent, given that Hadi effectively has been forced out of Yemen. His consent is not as robust as it would be if he remained in power in Yemen and was merely seeking outside assistance to suppress a modest rebellion. The situation on the ground clearly has moved beyond that. Nevertheless, there are a significant number of examples in the past decades in which states have acted militarily in support of and with the consent of leaders who have lost effective control of their countries. This is just another example to add to the pile.