By Jesse Calo
On February 25, President Biden ordered an airstrike in Syria against Iranian-backed militias in response to their rocketing of U.S. targets in Baghdad and Erbil from ten days prior. While President Biden asserted a constitutional basis for his authority to act, he also provided a basis for the strike under international law: that the inherent right to self-defense as proscribed by Article 51 of the U.N. Charter authorized his actions. President Biden did not present a novel theory of self-defense under Article 51; but, his theory is far from universally accepted in the international community. Regardless of whether the United States’ position is meritorious, though, this lack of international consensus on how to interpret Article 51 serves to weaken its intended purpose of functioning as an internationally recognized limit on the resort to force. I discuss two parts of President Biden’s legal theory on Article 51 to demonstrate this problem.
As a brief overview, Article 51 of the UN Charter explains that no member “shall impair the inherent right of individual or collective self-defense if an armed attack occurs against a Member of the United Nations[.]” This right to self-defense is an exception to the general rule proscribed by Article 2(4), that “[a]ll Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State, or in any other manner inconsistent with the Purposes of the United Nations.” In other words, use of unilateral force violates Article 2(4) unless the state using force was subject of an Article 51 “armed attack.”
However, according to the International Court of Justice (the court tasked with presiding over U.N. Charter disputes amongst states) (ICJ), a non-state-actor (NSA) can only commit an “armed attack” as defined by Article 51 if a state actor has “effective control” over the NSA. To have effective control, the state actor must provide direct and critical combat support to the NSA and exercises some tactical control over its operations. President Biden did not provide evidence of the extent to which Iran controls the targeted militia groups. Instead, the United States (as do many states on this issue), rejects the ICJ’s holding—asserting that NSAs may commit “armed attack[s]” within the scope of Article 51.
A second legal problem exists because President Biden conducted the airstrikes against the Iranian-backed militias within Syria. The ICJ has defined an “armed attack” as “action by regular armed forces across an international border” that is more substantial in scale and effect than a “mere frontier incident.” In other words, according to ICJ case law, if Syria had no “effective control” over these militias, one can argue that the U.S. not only violated Article 2(4) by “use[ing] force against the territorial integrity” of Syria; but moreover, that it committed an “armed attack” against Syria by engaging in more than a “frontier incident” within Syria’s borders.
Instead of working within this framework, President Biden presumably relied upon a theory the United States has forwarded in the past—the “unwilling or unable” test. Not universally accepted, the theory argues that State A may act in self-defense against an NSA within the territory of State B, if State B is “unwilling or unable” to take effective action against the NSA within its borders. While the theory solves the major problem of allowing an NSA to rely upon a state’s borders as a shield against defensive action, use of force based on ‘inability’ runs the risk of being pretextually asserted should the legal standard not be clearly defined.
But regardless of whether the United States’ theories on self-defense are correct, Biden’s airstrike demonstrates a broader structural problem. Because there is no authoritative voice on the meaning of the UN Charter, and because its text does not contemplate NSAs—states can rely upon this murky legal regime to mask unjust use of force in a veil of legality. Until the U.N. Charter is updated to confront the prevalence of NSAs in the 21st Century, the lack of consensus on the meaning and scope of Article 51 will serve to undermine the provision’s original purpose of preventing the unnecessary resort to force.
 Biden takes first military action with Syria Strike on Iran-backed militias, BBC News, Feb. 26, 2021, https://www.bbc.com/news/world-middle-east-56205056.
 Letter from President Joseph R. Biden, to Congresswoman Nancy Pelosi & Senator Patrick Leahy, A Letter to the Speaker of the House and the President pro tempore of the Senate Consistent with the War Powers Resolution, Feb. 27, 2021, https://www.whitehouse.gov/briefing-room/statements-releases/2021/02/27/a-letter-to-the-speaker-of-the-house-and-president-pro-tempore-of-the-senate-consistent-with-the-war-powers-resolution/?id=1564.
 Elena Chachko & Ashley Deeks, Which States Support the ‘Unwilling and Unable’ Test?, Lawfare Blog, Oct. 16, 2016, https://www.lawfareblog.com/which-states-support-unwilling-and-unable-test.
 See Dawood I. Ahmed, Defending Weak States Against the ‘Unwilling or Unable’ Doctrine of Self-Defense, 9 J. Int’l L. & Int’l Rel. 1, 6 (2013).
 U.N. Charter art. 51.
 U.N. Charter art. 2(4).
 There are more limits of the legality of self-defense than this; but, for the purposes of this blog post, this conception of legal self-defense is sufficient.
 Military and Paramilitary Activities in and Against Nicaragua (Nicar. v. U.S.), Judgment, 1986 I.C.J. Rep. 14 ¶ 105–15.
 See Brian Finucane, Fictitious States, Effective Control, and the Use of Force Against Non-State-Actors, 30 Berkley J. Int’l L. 35, 77–82 (2012).
 See Kenneth Anderson, Readings: Can Non-State Actors Mount an Armed Attack? by Kimberly N. Trapp, Lawfare Blog, June 14, 2014, https://www.lawfareblog.com/readings-can-non-state-actors-mount-armed-attack-kimberly-n-trapp.
 Military and Paramilitary Activities in and Against Nicaragua (Nicar. v. U.S.), Judgment, 1986 I.C.J. Rep. 14 ¶ 195.
 See id.; Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory, Advisory Opinion, 2004 I.C.J. Rep. 136 ¶ 139 (holding that Article 51 does not apply to NSAs without the effective control of a state actor).
 Chachko & Deeks, supra note 4.
 See Ashley S. Deeks, “Unwilling or Unable”: Towards a Normative Framework for Extraterritorial Self-Defense, 52 VA. J. Int’L L. 483, 512 (2012) (arguing for the need of a normative “unwilling or unable” test).