On Wednesday night, Saudi Arabia launched a military intervention in Yemen to stop Houthi advances through the country. Calling it “Operation Decisive Storm,” Saudi Arabia acted in coordination with a coalition including the United Arab Emirates, Qatar, Kuwait, Bahrain, Jordan, Morocco, Sudan, Pakistan and Egypt. Gulf Cooperation Council (GCC) members announced that the military action was taken in response to Yemeni President Hadi’s request to the leaders of Saudi Arabia, the United Arab Emirates, Bahrain, Oman, Kuwait and Qatar: “I ask you, based on the principle of self-defence in Article 51 of the Charter of the United Nations … to provide instant support by all necessary means, including military intervention to protect Yemen and its people from continuous Houthi aggression ….”

The White House has said the United States will provide “logistical and intelligence support” to the coalition’s military operations. “While U.S. forces are not taking direct military action in Yemen in support of this effort, we are establishing a Joint Planning Cell with Saudi Arabia to coordinate U.S. military and intelligence support,” reads a White House statement on American support to the operation in Yemen. Saudi Arabia is said to have “relied heavily on U.S. surveillance ­images and targeting information” in carrying out its military operation.

In aid to the Houthis, Iran has reportedly been providing weapons, financial support and military advice.

These developments elicit a number of questions under international law, some of which are briefly explored in this post.

1. Does the coalition’s military intervention constitute a lawful use of force?

Assuming President Hadi retains government authority to request the intervention, his consent precludes a violation of the prohibition of the use of force enshrined in Article 2(4) of the United Nations Charter. In contrast, Iran’s view is that the military operations have violated Yemen’s sovereignty. As Claus Kress explained in a post last month, “State practice does not support the proposition that a government invariably loses the power to invite armed assistance from abroad when it is confronted by internal violence reaching the level of a non-international armed conflict.” Of course, it is important to recall that no invitation to use force would absolve any intervening state of its relevant obligations under IHL or international human rights law.

2. Is Iran’s support of Houthi rebels lawful?

According to the International Court of Justice (ICJ) in the Nicaragua case, training, arming, equipping, financing and supplying rebel forces or otherwise encouraging, supporting and aiding military and paramilitary activities in and against a State is a breach of the obligation under customary international law not to intervene in the affairs of another State. The ICJ also found that providing weapons and training armed groups amounts to a violation of Article 2(4) of the UN Charter, while financial support does not. [See also Ryan Goodman and Michael Schmitt’s post, “Having Crossed the Rubicon: Arming and Training Syrian Rebels”]

3. Does international humanitarian law apply to the military intervention?

A separate crucial question about the airstrikes and supporting military activities is whether they fall under international humanitarian law (IHL). In order to determine if any of these supporting countries’ operations are governed by IHL, the following questions need to be answered:

a) Is there an armed conflict in Yemen?

Assuming President Hadi’s government authority remains valid, his consent to the military intervention would mean there is no international armed conflict between Yemen and the coalition forces, as there would be no “difference arising between two States and leading to the intervention of armed forces.” However, according to the contrary view of rebel leader Abdul-Malik al-Houthi, the military action aims to invade and occupy Yemen.

According to the test proposed by the Appeals Chamber of the International Criminal Tribunal for the Former Yugoslavia (ICTY) in the Dusko Tadic case, if Iran were to exercise “overall control” over Houthi rebels, then the armed conflict would be considered to be international between Iran and Yemen, with Houthi rebels effectively acting as agents of Iran. At this point, however, Iran’s influence over Houthi rebels does not seem to meet this test. (If the relationship does not meet the “overall control” test, there is no reason to speculate whether it meets the so-called “effective control” test, which sets an even higher threshold for the purpose of attributing state responsibility.)

As for fighting between forces loyal to President Hadi and Houthi forces, it would appear that the intensity of violence and their level of organization have both met the threshold for a non-international armed conflict (NIAC), as the International Committee of the Red Cross (ICRC) has found.

b) Are the United States and other countries parties to the pre-existing NIAC in Yemen?

Depending on their respective forms of support, it could be, for example, that the United States and Iran might find themselves as opposing parties to the same armed conflict. While it may still be too early to determine whether any of the supporting states has become a party to the NIAC in Yemen, as I stated in a post earlier this year, two approaches may assist in determining, from the facts, whether a country has become a party to a pre-existing NIAC for the purposes of applying IHL: (1) the “support-based approach” proposed by ICRC Legal Adviser Tristan Ferraro; and (2) the concept of “co-belligerency” under the international law of neutrality.

According to the support-based approach, a state would be a party to a pre-existing NIAC, and therefore be bound by IHL, when the following four conditions are met:

  • There is a pre-existing NIAC ongoing in the territory in which the country’s forces intervene;
  • The intervening country’s forces undertake actions related to the conduct of hostilities in the context of the pre-existing NIAC;
  •  The intervening country’s forces’ military operations are carried out in support of a party to the pre-existing NIAC;
  •  The intervening country’s actions are undertaken according to an official decision to support a party involved in the pre-existing NIAC.

Ferraro proposes that “[w]ar-sustaining activities such as financial support, or the delivery of weapons/ammunition to a party to the conflict, should be regarded as a form of indirect involvement in hostilities” that would not suffice for the purpose of applying the second criterion. He adds, “[w]hen these weapons/ammunition are not immediately used by the supported party against its adversary, the causal relationship between their delivery and the harm inflicted by the supported party while using them is too indirect for such support to be deemed an integral part of the collective conduct of hostilities.” Instead, support such as transporting armed forces of the supported party to the front line, or providing planes for refueling of jetfighters, would make the supporting forces party to the NIAC. He explains, “there must be a close link between the action undertaken by multinational forces and the harm caused to one of the belligerents by specific military operations undertaken by the opponent.“

As for the concept of “co-belligerency,” it is a concept of the international law of neutrality, which regulates relations between states that are participating in an international armed conflict (belligerent states) and those that are not (neutral states). A neutral state becomes a belligerent by: (1) declaring war; (2) participating in hostilities to a significant extent; or (3) through systematic or substantial violations of its duties of impartiality and non-participation. These can consist of supplying warships, arms, ammunition, military provisions or other war materials to a belligerent, engaging one’s own military forces, supplying military advisors to a party to the armed conflict, or providing or transmitting military intelligence, among other acts.

Author Morris Greenspan has proposed that a co-belligerent under this approach is a “fully fledged belligerent fighting in association with one or more belligerent powers.” According to the UN Special Rapporteur on extrajudicial, summary or arbitrary executions, “Co-belligerency is a concept that applies to international armed conflicts and entails a sovereign State becoming a party to a conflict, either through formal or informal processes. A treaty of alliance may be concluded as a formal process, while an informal process could involve providing assistance to or establishing a common cause with belligerent forces.” In a 2004 Memo, the U.S. Office of Legal Counsel explored the type of participation required for a country to qualify as a co-belligerent in occupied Iraq:

a State that sends military forces to assist in rounding up Baathist remnants and imposing general security in Iraq, and especially one that participates in hostile activities in Iraq, will engage in conduct properly characterized as belligerent. In sum, the determination whether a State is a “co-belligerent” by virtue of its participation in the occupation of Iraq turns on whether the participation is closely related to ‘hostilities.’ (emphasis added)

Under the co-belligerency approach, a state’s systematic or substantial supply of war materials, military forces, or military intelligence in association, cooperation, assistance or common cause with another belligerent would make it a co-belligerent. Of course, transposing the law of neutrality onto NIACs remains a point of controversy among international legal experts.

Should the United States or any other supporting country become a party to the pre-existing NIAC in Yemen, then IHL will govern their actions in the conduct of hostilities (and so far, the military intervention has consisted of airstrikes only). Other implications may include widening the geographic scope of the NIAC to these other parties’ territory, and rendering members of the armed forces of these states lawful targets of attack. Under IHL, parties to the conflict must distinguish between civilian objects and military objectives and may only direct attacks against military objectives. They must also distinguish between civilians and combatants and may only direct attacks against combatants. These are members of state armed forces as well as members of an organized armed group of a party to the conflict. Debate persists as to whether members of an organized armed group comprise only those individuals who have a “continuous combat function” or rather any member of the group. Persons who are not considered members of an organized armed group of a party to the conflict are civilians and are entitled to protection against direct attack unless and for such time as they directly participate in hostilities.