Never-ending War: The ‘Legalization’ of US Military Intervention from Vietnam to Iran

[Claire Beutter is a PhD Candidate in International Law at the Geneva Graduate Institute.]

Writing for the New York Times in 1965, Hans Morgenthau expressed his hope that the “vaunted pragmatism and common sense of the American mind” would correct the dangerous misconceptions driving US involvement in Vietnam, “before they lead us from the blind alley in which we find ourselves today to the rim of the abyss.” Sixty years later, the abyss draws closer, and a “long-overdue lesson” about foreign military intervention remains to be taught. From Venezuela to Iran, the Trump administration has continued a longstanding practice of coercive intervention abroad, underwritten by an expansive interpretation of Article 51 of the UN Charter. Despite President Trump’s disavowal of international law in favor of “his own morality”, the Office of the Legal Adviser continues to offer perfunctory legal justifications for US conduct, exemplified by the Office of the Legal Adviser’s “defective and overly permissive” statement on Operation Epic Fury and international law. Faced with a concept of self-defense that has been stretched beyond meaning, prompting reflections on the erosion of authoritative legal meaning and what is left of international law, it is worth considering how we arrived here. 

Rather than a Trumpian anomaly, the US statement on Operation Epic Fury can be read as part of a longer tradition of what Ríán Derrig terms an “American anti-formalist legal practice.” Myres McDougal, the father of American anti-formalism, wrote during the late 1950s and early 1960s, a time of “anxiety” about the future of international law, coupled with a strong sense of prospective US power in shaping this future. Far from a peripheral actor, McDougal sat at the center of international law scholarship and practice in the US, dialoguing comfortably with sibling movements such as Morgenthau’s international relations realism. Derrig classifies McDougal’s approach to international law as “anti-formalist”, part of a broader reaction against the logical reasoning of classical international law. At the core of this anti-formalist approach was a conception of law as a continuous process of claims and counterclaims, rather than a static body of rules. In lieu of formalistic readings based on “plain meaning’” McDougal advocated for legal texts to be interpreted by reference to their context, emphasizing the inescapable role of values in determining law. 

Although anti-formalism in some instances served weaker parties resisting the interests of the powerful, McDougal’s brand of anti-formalism was more often invoked in support of power. Nowhere is this clearer than in the context of US military intervention: by blurring the categorical distinction between war and peace, McDougal, alongside Florentino Feliciano “presciently laid the conceptual foundation for the idea that the United States would always be at war”. They argued that violence, or “coercion”, was best understood as a continuum, expanding the interpretive space around the law on the use of force. Such framing was not unique in US legal scholarship at the time: Derrig notes Abram Chayes’ parallel preoccupation with the dissolution of the traditional dichotomy between war and peace into a reality of “total war”. Applying this frame to Article 51 of the UN Charter, McDougal and Feliciano argued for the legality of pre-emptive self-defense, a reconceptualization of international law that “sowed seeds that would bear a harvest of arguments advanced by students of McDougal’s, particularly over the legality of the Vietnam War, and later still over the legality of American actions in Nicaragua.” 

The Office of the Legal Adviser’s memorandum on the Legality of United States Participation in the Defense of Viet Nam is a marked example of the institutionalization of this anti-formalist approach. Eschewing a formalist interpretation of “armed attack”, the memorandum contextualized Article 51 within a longer tradition recognizing the “inherent” right of self-defense, framing Article 51 as a “saving clause” for this right. In doing so, it recast US intervention in a fundamentally internal conflict as “collective self-defense.” Faced with circumstances in which “an ‘armed attack’ [was] not as easily fixed by date and hour as in the case of traditional warfare,” the memorandum nonetheless concluded that the infiltration of North Vietnamese forces into South Vietnam “clearly constitutes an ‘armed attack’ under any reasonable definition.” Implicit in this conclusion was the inseparable tie between “reasonableness” and the perceived security needs of the US, a relation that McDougal had pioneered alongside Norbert A. Schlei in an earlier article on the thermonuclear weapon tests on the Marshall Islands. 

Six decades later, the seeds sowed by McDougal’s anti-formalism have firmly taken root. On April 21, the Office of the Legal Adviser, headed by Reed Rubenstein, published a statement on Operation Epic Fury and international law. In contrast to IHL-grounded evaluations, which mark Operation Epic Fury as itself triggering the advent of an international armed conflict between the US, Israel, and Iran, the statement justifies US-Israeli combat operations in Iran as an act of collective self-defense in light of an “ongoing international armed conflict with the Islamic Republic.” In doing so, the statement builds on a series of letters transmitted by the US to the Security Council dating back to February 27, 2021, most notably one dated June 27, 2025 addressing the legality of Operation Epic Fury. By invoking the “inherent” right of self-defense “as reflected in Article 51”—a move initially articulated in the Vietnam memorandum—the letter decouples the concept of self-defense from the text of Article 51, opening a wider interpretive space. Like McDougal and Feliciano, it rejects a formal division between peacetime and wartime, contextualizing US intervention as part of “a very long pattern of [Iranian] aggression and provocation against the United States and its allies and partners.” In lieu of a formalist textual analysis, the letter adopts a value-oriented, contextual lens, concluding that Operation Epic Fury represents “necessary and proportionate action” taken by the US to defend Israel and protect its own “security, citizens, and interests, consistent with Article 51 of the Charter.” Meanwhile, Iran’s argument that the US-Israeli use of force against it violates international law is discredited as “cynical lawfare”, a formalistic foil to the letter’s contextual assessment of “necessity”.

The Legal Adviser’s statement goes further, arguing that US actions “could equally have been considered part of an ongoing international armed conflict between Iran and the United States itself, in which the United States was exercising its own, individual right of self-defense.” On this reading, previous periods of active hostilities—including Operation Midnight Hammer, a series of US strikes against Iranian nuclear facilities that took place during a twelve-day conflict between Iran and Israel in June 2025—are not isolated armed conflicts breaking the peace, but accumulated attacks in a single ongoing conflict between the US and Iran. The statement fails to delineate the precise boundaries of this conflict, noting only that it “predate[s]” Operation Epic Fury and “continues to this day.” By asserting that Iran “has regularly attacked the United States, its interests, and its allies” since its founding in 1979, the statement implies a longstanding, existential conflict, with clear parallels to McDougal and Feliciano’s continuum of coercion. Like the June 27 letter, the statement privileges context in its legal interpretation: any “serious” legal assessment of US intervention, it declares, must be “anchored in the relevant material facts.” Per the anti-formalist tradition established by McDougal, the interpreter—in this case, the Trump administration—retains broad discretion to identify which facts are “relevant” or “material.” 

Some may take the statement’s sweeping claims and conclusory analysis as further evidence that the United States has abandoned any pretense of respect for international law. Yet it may also be read as a continuity, the natural product of decades of anti-formalist precedent. Grounded in McDougal’s approach, which meticulously redrew the boundaries of jus ad bellum, contemporary US argumentation benefits from an unbounded interpretive field. By invoking a free-standing “inherent” right to self-defense, untethered from the understanding of self-defense enshrined in the UN Charter, the US has dissolved the formal limits of the law. A blurred line between peace and war underpins a narrative of never-ending conflict, an omnipresent threat that can be invoked to justify coercive military action in the name of national security. Put simply, years of accumulated practice invoking Article 51 expansively, contextually, and based on self-judged security interests empowers the US to assert “necessity” as a practically self-evident conclusion today. 

This is not to suggest that the US’s expansive interpretation of jus ad bellum has gone unchallenged, either within the US or internationally. Writing for the Yale Law Journal in 1966, Richard Falk called for an “examination of the increasingly imperial role of the United States in international society”, suggesting that “[p]erhaps we will discover the relevance of international law to the planning and execution of foreign policy as well as to its justification.” The 1986 Nicaragua judgment by the ICJ similarly rejected the US’s interpretation of self-defense, constructing a high threshold for an armed attack and ruling that a third-party state could not unilaterally decide to act in collective self-defense without a request by the affected state. Many states have attempted to hold the line by openly contesting the US’s expansive interpretation of self-defense and declaring US intervention unlawful. Other states, however, have adopted anti-formalist modes of interpretation to justify their own conduct. 

As regards Operation Epic Fury, over 100 US-based international law experts have affirmed that it violated the UN Charter, while the Legal Adviser’s proffered justification has drawn criticism for being “both legally unpersuasive and analytically confused.” Formal legal critique remains a valuable means of condemning unlawful conduct, holding the line, however thin, against the complete collapse of shared legal norms. Yet it would be “myopic” to view self-defense in purely legal terms; we must also reckon with reality of an increasingly anti-formalist international legal order. Expansive conceptions of “necessity” and “self-defense” are readily available to support the self-serving interpretations of international law put forth by powerful states. Anti-formalism has become a defining characteristic of what Antony Anghie terms a “Hobbesian universe”, defined by the subordination of the UN system and the “unilateral use of force ostensibly in self-defense.” Faced with this state of affairs, international lawyers may be inclined to ask how the formal limits of jus ad bellum, once dissolved, can be restored. Yet this framing overlooks the consistent failure of those same limits to meaningfully constrain violence by the US and other powerful states since the inception of the contemporary international legal order. Rather than a return to the familiar, then, it may be time for a more fundamental reckoning with what just, effective constraints on the use of force would require.