Australia, New Zealand, and Operation Poseidon Archer: Some International Law Questions

Written by Ashley (Ash) Stanley Ryan



On 12 January 2024, the United States and its coalition partners launched a series of strikes against Houthi (Ansarallah) positions in Yemen, as part of “Operation Poseidon Archer”. Those strikes were justified as acts of individual and collective self-defence, in response to attacks by Ansarallah against civilian and naval vessels in the Red Sea. Australia provided support for these strikes; on 24 January 2024, New Zealand announced that it, too, would provide support for future strikes against Ansarallah. In this piece, I analyse whether New Zealand and Australia have become parties to a non-international armed conflict (NIAC); the legal basis underlying the use of force; and what the situation tells us about the difference between international law and an international rules-based order (IRBO).

Operational support and belligerent status in a NIAC

First, I consider whether New Zealand and Australia are parties to a non-international armed conflict (NIAC) in the Red Sea. This is important for two reasons:

  1. If there is a NIAC, then New Zealand and Australia are likely belligerents under what the International Committee of the Red Cross calls the “support-based approach”.
  2. If there is no NIAC, operational support may nonetheless amount to participating in a use of force, each time that it is an integral part of an attack. This is a natural extension of the support-based approach.

The Criteria for a NIAC vary depending on states’ treaty obligations. For New Zealand and Australia, there are two relevant tests:

In the case of Ansarallah, the organisation requirement is clearly met. The remaining question is whether the hostilities have exceeded a minimum level of intensity.

There are two sets of relevant hostile acts: attacks by Ansarallah, and attacks against Ansarallah. Ansarallah’s strikes have been directed primarily against merchant vessels, with a limited number targeting military assets. The main consequence of these attacks, besides damage, has been a reduced willingness by maritime operators to transit the Suez Canal; they have been described by Ansarallah as intended to reduce support for Israel. Responses as part of Operation Poseidon Archer have taken the form of targeted air and missile strikes within the territory of Yemen, against Ansarallah installations.

Attacks on warships are capable of triggering the right to self-defence, and could result in a situation exceeding the minimum intensity of hostilities required to trigger a NIAC. It is not established in law whether attacks on merchant vessels, particularly those of third states, are capable of triggering either the law of self-defence or the law of NIAC, especially when those attacks are scattershot and the flag states concerned are not engaging in armed activities.

Does this exchange exceed a minimum level of violence? I argue that it does—at least, after the first targeted strikes by coalition partners. Responding to acts of violence, against state naval vessels, with repeated air strikes and missile strikes is clearly beyond a minimum level of violence, as is the continuation of attacks against naval vessels by Ansarallah. Factually, there appears to have been a NIAC since, at latest, January 12 2024, when Operation Poseidon Archer commenced.

The next question, then, is whether New Zealand and Australia are co-belligerents to this NIAC.

The support-based approach

The support-based approach, as termed by the ICRC, is a tool for identifying the states party to contemporary conflicts. It is concerned with a state’s factual contribution to the conduct of hostilities: it is related to the issue of direct participation in hostilities, and is closely linked to both the law of international responsibility and the law of neutrality. Under the support-based approach, a state becomes a party to a conflict where its forces, in support of one existing party, either “directly damage” the opposing party or “directly undermine” its military capabilities—even if that effect is only realised in conjunction with other acts, undertaken by the supported party. This phrasing captures conduct such as targeting assistance, but not logistical support or the lawful sale of arms.

New Zealand and Australia’s support consists of providing personnel who—at least, in New Zealand’s case—seem to be directly involved in targeting decisions. Targeting is a crucial component of any military operation; providing “precision targeting” support, including the provision of intelligence to be immediately used in the conduct of hostilities, is the textbook example of support amounting to co-belligerency. Australia has not stated the precise contribution its personnel are making.

On the information publicly available, it appears that New Zealand has been party to a NIAC since at least 24 January 2024, when it announced the deployment of Defence Force members to undertake “precision targeting”. Depending on the nature of Australia’s contributions it may have been a party as early as 12 January 2024, or it may have become a party after this time through a support-based approach, or it may never have become a party.

Reasonable minds may, of course, differ on whether the situation reaches the intensity threshold for a NIAC. Even if there is no NIAC, I suggest that the support-based approach can be analogised to the use of force: through providing precision targeting assistance since 24 January, New Zealand (and possibly Australia) is making an essential contribution to repeated acts amounting to the use of force against a third state’s territory. This approach, and the support-based approach, is consistent with Article 16 of the Articles on State Responsibility regarding aid or assistance with the commission of an act.

In any case, whether there is a NIAC or not, force is being and has been used; this raises the question of its legality.

Legal justifications for the use of force

In modern international law, the use of force is tightly constrained both by treaty and custom.  The (uninvited) use of force against the territory of another state is lawful only either in self-defence against an armed attack, or with the authorisation of the UN Security Council (UNSC).  There has been no UNSC authorisation; the relevant resolution, S/Res/2722, simply acknowledges the right for a state to defend its own vessels from attack, “including those that undermine navigational rights and freedoms”. This is not an authorisation of the use of force, nor has any state invoked the resolution as the legal basis for their acts. Instead they have almost exclusively referred to their inherent right of self-defence. I here assume that the United States, New Zealand and Australia have adopted Bethleham’s “unable and unwilling” doctrine to justify acts of self-defence against non-state actors on another state’s territory; this doctrine is highly controversial, but without it, the non-consented use of force on the territory of a third state would be unlawful. It presumably therefore underlies the legal advice provided to all states involved in the coalition.

New Zealand and Australia, specifically, have invoked collective self-defence. New Zealand and Australia both signed a joint statement by coalition partners on 12 January, which invoked collective self-defence and stated that Australia provided “support” for strikes; and on 23 January New Zealand’s Prime Minister, its Defence Minister (who is also the Attorney-General), and the Foreign Minister stated that the NZDF deployment was an act of “collective self-defence of ships”.

Collective self-defence is a complex concept. Not only do the core limbs of self-defence—an armed attack must have occurred, and the response must be both necessary and proportionate—apply, but according to the ICJ there are two additional criteria. A state must declare that it was the victim of an armed attack; and it must request assistance. Whether the declaratory obligation is an actual customary rule is contested; but the requirement that assistance be requested is generally accepted.

A request for assistance naturally presupposes that an armed attack has occurred. This is the key challenge for individual or collective self-defence claims concerning civilian vessels—especially civilian vessels under the flag jurisdiction of a third state. It is highly tenuous, for example, to suggest that an attack on a vessel flagged to Belize but owned by a British-based company, incorporated in the Marshall Islands, constitutes an armed attack on the United Kingdom. Even if it did constitute an attack on Belize, they would need to request collective self-defence. This challenge is likely part of the reason why both the US and UK’s article 51 letters refer only to attacks against their naval vessels and invoke individual, not collective, self-defence.

This reference to individual self-defence should be read alongside the lack of any clear, public request for aid. While there is no requirement that requests for self-defence be made publicly, it is beneficial for everyone if they are. When requests are made privately, responding states risk being accused of violating the prohibition against the use of force, since there may be no clear legal ground upon which their action is based—it may not even be clear what state made the request.

Because the factual situation is unclear, while preparing this piece I requested from New Zealand’s Foreign Ministry two pieces of information: whether a request for collective self-defence was made, and whether New Zealand considers that attacks on civilian vessels flagged to third states are capable of triggering collective self-defence. The Ministry replied that on January 10, an informal note was produced titled “United States request for New Zealand support for action against the Houthi”. As concerns attacks on civilian vessels, the Ministry refused to release any additional information. Instead, it repeated the phrase “collective self-defence of ships in the Middle East, in accordance with international law”.

The Ministry’s response re-emphasises why it is crucial to identify the precise legal justification for acts of collective self-defence, and why requests for collective self-defence should be made publicly. It is not entirely clear whether a request of “support for action” is a request for collective self-defence. It is also not clear whether the Ministry believes that the triggering armed attack is the attacks on civilian vessels flagged to third states, or the attacks on American naval vessels.

On the assumption that collective self-defence wastriggered following an armed attack against and support request from the United States, the response must be necessary and proportionate with relation to the threat to the requesting state. This is important for two reasons:

  1. Self-defence is not a carte blanche to take wide-reaching or opportunistic military action. It must be in response to the triggering armed attack: that is, the targeting of American naval vessels in the Red Sea.
  2. Justifications that go beyond the immediate threat to American naval vessels are political, not legal, and risk undermining the legitimacy of an act of self-defence.

Action which goes beyond the threat, as identified in 1), risks transforming an act of self-defence into an unlawful use of force. This is why it is so important that states be clear on precisely what armed attack they are defending against. Again, it is not clear whether New Zealand or Australia believe they are acting in collective self-defence of American naval vessels, or if they believe that there is a free-standing right of collective self-defence related to attacks on civilian vessels, even in the absence of a request from one or more of those vessels’ flag states.

Support for military operations in the Red Sea has not only been justified as self-defence—in fact, it has progressively strayed from this justification. The 3 February and 24 February statements by coalition partners make no reference to self-defence; they rather emphasise the importance of “freedom of navigation and international commerce” and “holding the Houthis accountable” for their attacks. The 24 February statement specifically referenced four attacks against “American and British-owned” vessels. None of the attacked vessels are registered under the flag state jurisdiction of either the US or UK, or any other coalition partner. This language suggests that the ongoing operation is straying from its strict legal basis—self-defence of American and British naval vessels—and into an amorphous, legally uncertain conflict in pursuit of the free flow of international trade. If this holds true, then the claim of self-defence will inevitably expire and the use of force will become unlawful.

Self-defence, including collective self-defence, also triggers a procedural obligation of reporting measures taken to the UN Security Council. Past practice, including regarding coalition operations against Daesh, indicates that all states which integrally contribute to a use of force—including in collective self-defence—are under a procedural obligation to report their actions. New Zealand and Australia have not, to date, issued an article 51 letter. While this does not preclude collective self-defence, it once again makes it difficult to assess the legality of the claim.

International law, or an IRBO?

So, then, it appears that New Zealand (and possibly Australia) are involved in a NIAC; and that their involvement is premised on a somewhat unclear interpretation of the law of collective self-defence; and that rhetoric surrounding the NIAC has drifted from self-defence to vague questions of accountability and freedom of commerce. I suggest that this drift, and much of the ambiguity surrounding New Zealand and Australia’s legal position, is symptomatic of the difference between a system based in international law and an international rules-based order (IRBO).

International law is flexible and open-textured: a wide range of actions may be justifiable within its frame. However, actions must be justified either with reference to the law, or as otherwise unregulated or non-opposable. In areas such as the use of force, the law is less open-textured; the primary areas of contention are the question of whether non-state armed groups can conduct an armed attack, and whether there is, as a matter of custom, an emerging “unable or unwilling” rule authorising self-defence against actors located within the territory of another state.

By contrast, claims which appeal to, for example, “defending freedom of navigation”, or protecting “international commerce” are not appeals to the law. Nor, despite their appearance, are assertions that company ownership of a civilian vessel is somehow analogous to flag state jurisdiction. Such arguments use legal and law-adjacent terminology, but they are not legal; they are used to argue for political legitimacy, not legality. They are appeals to an IRBO. This distinction is important because an IRBO is even more open-textured than international law. Its contents are wholly indeterminate; it can contain and legitimise essentially any course of action, provided that the action is justified as necessary to ensure respect for the order. There is consequently a much greater latitude in an IRBO approach to, for example, the use of force beyond the generally accepted forms of self-defence, and a much greater deference to policy considerations. This can lead to situations where an act is politically legitimate, but nonetheless internationally wrongful.

New Zealand and Australia have, historically, used the term IRBO as a synonym for international law. The Red Sea intervention perhaps represents the beginning of a shift in approach. New Zealand and Australia began their engagement by mixing legal justifications with appeals to an IRBO; subsequent statements by the coalition, the text of which New Zealand and Australia have presumably contributed and consented to, have almost exclusively appealed to an IRBO. I suggest that there are two major concerns which arise from this shift. First, it may undermine the reputation both states enjoy of being responsible, predictable actors. This is because it suggests that they are willing to risk committing internationally wrongful acts, if doing so is beneficial to the national interest and can be politically justified. Second, the shift risks the wider integrity of international law: when a group of states, through their appeals to an IRBO, seek to legitimise otherwise internationally wrongful or legally dubious acts, they encourage other states to do the same. This undermines universality, and may also result in a proliferation of parallel, competing IRBOs, each of which seeks to provide different groups of states with policy-based legitimisation for legally questionable conduct. This risk is especially pronounced when an IRBO is being deployed to justify departures from the most fundamental elements of modern international law—such as, for example, when a state asserts that its national interest or policy considerations legitimise a broadened interpretation of the inherent legal right of self-defence.

Ashley (Ash) Stanley Ryan
+ posts

Ashley (Ash) Stanley-Ryan is a Teaching Assistant at the Geneva Academy and a PhD candidate in international law at the Geneva Graduate Institute, and an enrolled Barrister and Solicitor (New Zealand). His doctoral research examines the role of the United Nations Secretariat in the development of international law, particularly law related to international peace and security. He holds a LLB and a BA from Te Herenga Waka, and an LLM summa cum laude from the Geneva Graduate Institute. Ash’s  professional experience includes working at a magic circle law firm, the UN, the ICRC, and New Zealand’s delegation to the United Nations in New York. He was one of the two centenary recipients of the prestigious diploma of the Hague Academy of International Law, in public international law.

Get in Touch with the Editor

ANZSIL Perspective is pleased to hear from its readership and answer any questions from prospective contributors. We aim to respond within three business days.