Can Israel Target Iran in Response to Houthi Attacks? Exploring the Threshold of “Substantial Involvement”
Following the Houthi missile attack on Ben Gurion Airport on Sunday, May 4, 2025, Israeli Prime Minister Benjamin Netanyahu vowed to retaliate not only against the Houthis, but also against their “patron,” Iran, declaring that such action would take place “at a time and place of our choosing.” In a similar vein, U.S. President Donald Trump asserted, “Every shot fired by the Houthis will be looked upon, from this point forward, as being a shot fired from the weapons and leadership of IRAN, and IRAN will be held responsible, and suffer the consequences, and those consequences will be dire.”
Iran, for its part, denied ordering the attacks. Its Foreign Minister stated that “the actions of the Yemenis in support of the Palestinian people were an independent decision stemming from their feeling of solidarity.” The Iranian Defence Minister however warned that Iran would respond to any attack against it.
This latest incident is part of a broader pattern of Houthi attacks targeting Israel and other States, to which those States have responded with force in self-defence. The Houthis, along with Hamas and Hezbollah, are widely regarded as members of the so-called “axis of resistance,” which is understood to form part of Iran’s “forward defence” or “mosaic defence” strategy.
In response Israel has hit Houthi targets in Yemen. This post examines the question whether Israel may lawfully respond to the Houthi attacks by using force in self-defence against Iran. The analysis will focus on Article 3(g) of the UN General Assembly’s Definition of Aggression which the International Court of Justice (ICJ) invoked in its Paramilitary Activities judgment to define the concept of an “armed attack” for the purposes of the right of self-defence under Article 51 of the UN Charter (para. 146). The post concludes that Iran’s substantial involvement with the Houthis renders it the author of their attack and a legitimate target of action in self-defence.
Article 3(g) of the General Assembly’s Definition of Aggression
Among the acts of aggression enumerated in the General Assembly’s Definition of Aggression, Article 3(g) refers to,
The sending by or on behalf of a State of armed bands, groups, irregulars or mercenaries, which carry out acts of armed force against another State of such gravity as to amount to an actual armed attack conducted by regular forces, or its substantial involvement therein.
Although Article 3(g) addresses armed attacks carried out by non-State actors, it functions as a primary rule that defines State authorship of an armed attack, rather than as a secondary rule of attribution that imputes the conduct of non-State actors to States.
The provision articulates three distinct modes through which a State may be considered the author of an armed attack: i) the sending of non-State actors to carry out an armed attack on the territory of another State; ii) non-State actors committing an armed attack on behalf of a State; and iii) a State’s substantial involvement in the activities of non-State actors who carry out an an armed attack.
I will now consider each mode in turn and apply them to the relations between Iran and the Houthis.
The “Sending” and “On Behalf” Modes of Committing an Armed Attack
The notion of “sending” non-State actors, as a mode of authorship of an armed attack under Article 3(g) of the Definition of Aggression implies, according to the International Fact-Finding Mission on the Conflict in Georgia, that the non-State actors in question function as de jure or de facto organs of a State (p. 258-60). However, “sending” may also be understood to include situations in which a State instructs or directs a non-State actor to commit an armed attack, as stipulated in Article 8 of the International Law Commission’s Articles on State Responsibility (ASR).
Similarly, committing an armed attack “on behalf of a State” may be interpreted to mean that the non-State actor is a de facto organ of the State (para. 109), has been instructed by the State, or is acting under the State’s effective control (p. 258-60), again in line with Article 8 ASR. It is evident that the interpretation of these terms remains unsettled, and that there is a degree of overlap between these two modes of State authorship of an armed attack and the attribution criteria under the law of State responsibility.
Practically, the legal consequence is the same: the State that sends non-State actors to carry out an armed attack, or on whose behalf non-State actors carry out an armed attack, may become the lawful target of self-defence action, as does the State to which a non-State armed attack is attributed. Conceptually, however, there is a difference. Under Article 3(g), the State is deemed the direct author of the armed attack. In contrast, attribution involves a two-step process: the commission of an armed attack by a non-State actor, followed by its imputation to a State through established attribution rules.
Applying these criteria to the Houthi attacks, and based on the evidence currently available, it cannot be said that the Houthis are de jure or de facto organs of Iran, nor that they were ordered or directed by Iran to carry out attacks against Israel. Accordingly, Israel would not be entitled to invoke the right of self-defence against Iran on that basis.
The “Substantial Involvement” Mode of Committing an Armed Attack
The third mode by which a State may commit an armed attack under Article 3(g) of the Definition of Aggression is through substantial involvement. This concept was invoked in the Paramilitary Activities case. The ICJ however rejected without explanation the United States’ argument that Nicaragua’s alleged provision of weapons, logistical aid, and other forms of support to Salvadoran rebels amounted to an armed attack (paras. 195, 226-31). Instead, it focused exclusively on the “sending” and “on behalf” prongs of Article 3(g), leaving the scope and meaning of substantial involvement largely unexplored. Judges Schwebel and Jennings strongly criticised this in their dissenting opinions. Judge Jennings asked “what it is, short of direct attack by a State’s own forces, that may not be done apparently without a lawful response in the form of … self-defence” (p. 543-44).
Because Article 3(g) of the Definition of Aggression reflects customary international law (para. 195), all its elements must be given effect. Substantial involvement refers to a State’s complicity in an armed attack through various forms of assistance to a non-State actor. It is a reformulation of previous proposals which used terms such as “encouragement,” “instigation,” “assistance,” “participation,” and “acquiescence” (see also here). Article 3(g) thus represents a special and indeed a primary rule of complicity in non-State armed attacks leading to authorship of the armed attacks in contrast to Article 16 ASR which is a general but derivative rule of complicity.
I will now explain what type and level of assistance amounts to “substantial involvement” as well as what mental element is required. In the analysis that follows I draw on States’ views during the negotiations of the Definition of Aggression and also Article 16 ASR which exhibits primary as well as secondary characteristics (p. 415 and here p. 259).
Regarding the type of assistance, it may include the provision of training, funding, equipment, intelligence, and military or technical advice. In some circumstances, it may also extend to omissions, such as a State’s toleration of non-State actors operating from its territory or failing to prevent such operations. In short, any type of assistance regardless of quality or quantity or whether it is long or short term will satisfy this requirement provided it makes a substantial contribution as discussed below.
Whether ideological assistance—broadly understood—falls within the scope of “assistance” for the purposes of substantial involvement is context-dependent. In my view, ideological support may qualify as assistance where it plays a motivating or facilitating role in the commission of an armed attack. It is interesting to note that the Iranian Foreign Minister justified the Houthi attacks on their feeling of solidarity with the Palestinians.
Whereas the scope of provided assistance is broad, in order to trigger Article 3(g), this must make a substantial contribution to the armed attack carried out by the non-State actor. It need not be essential to the attack in the strict sense that the attack would not have occurred but for the assistance. If that were the case, the standard would amount to effective control over the attack for the purposes of Article 8 ASR, or fall within the “on behalf of” criterion discussed earlier.
Conversely, the assistance must not be merely marginal or incidental. Rather, it must be of a kind that bears on the non-State actor’s decision to carry out the attack and its execution. In other words, the assistance should facilitate the commission of the armed attack in a meaningful way.
Third, the assisting State must deliberately provide such assistance to the non-State actor. However, it is not necessary to know that the non-State actor will carry out a specific attack; rather, it is sufficient that the State has knowledge that the non-State actor is determined to carry out armed attacks using the assistance provided.
Applying the above framework to the relationship between Iran and the Houthis, it is evident that while the Houthis present themselves as independent actors, not subordinate to Iran, there exists a clear ideological and political alignment between the two, including a shared worldview with respect to Israel. Iran is reported to provide the Houthis with training, funding, oil, intelligence, and military support, including the supply of drones and ballistic missiles, and has also allegedly advised the Houthi military command (see here, here, and here).
This long-term and multi-dimensional assistance has significantly enhanced the Houthis’ operational capabilities. Moreover, Iran provides the assistance intentionally and with knowledge of the Houthis’ pattern of conduct, including their armed attacks. On this basis, Iran’s support meets, in my opinion, the threshold of substantial involvement under Article 3(g) of the Definition of Aggression, rendering Iran the author of the armed attack and entitling Israel to use defensive force against Iran.
Conclusion
The post considers “substantial involvement” stipulated in Article 3(g) of the Definition of Aggression as a mode of State authorship of an armed attack by looking at the relationship between Iran and the Houthis, who continue to attack Israel as well as other States. In my opinion, it is both timely and necessary to revisit the concept of substantial involvement as a mode of State authorship of an armed attack. Non-State actors today can be very powerful and control territory whereas their relationships with States are no longer necessarily characterised by dependency or hierarchical control reflected in traditional attribution criteria. The “substantial involvement” standard better reflects contemporary relations between States and non-State actors characterised by informal alliances and diverse forms of mutual support which allows them to maintain some distance and plausible deniability.
Secondly, substantial involvement serves to preserve the integrity of the jus ad bellum by ensuring that States which enable or empower non-State actors to carry out cross-border attacks can be held accountable. It recognises that complicity in the use of force may be just as harmful as direct participation and guards against the right of self-defence being nullified by the narrow standards of attribution.
Third, and related to the above, the substantial involvement standard lowers the evidentiary threshold required for holding States responsible for an armed attack. While proving attribution requires very strong evidence, substantial involvement focuses on the existence of a meaningful and sustained alignment between the State and the non-State actor engaged in armed attacks.
That said, substantial involvement is not without risk. Its broader and more flexible formulation may be vulnerable to abuse. Nevertheless, as a legal construct, it offers a more responsive framework for assessing the use of force by non-State actors in cases where State support is substantial but does not amount to direct control.
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Nicholas Tsagourias is Professor of International Law at the University of Sheffield, UK.
The views expressed are those of the author, and do not necessarily reflect the official position of the United States Military Academy, Department of the Army, or Department of Defense.
Articles of War is a forum for professionals to share opinions and cultivate ideas. Articles of War does not screen articles to fit a particular editorial agenda, nor endorse or advocate material that is published. Authorship does not indicate affiliation with Articles of War, the Lieber Institute, or the United States Military Academy West Point.
Photo credit: Voice of America
