Through the Looking Glass, Again: The Caroline Standard and a General Framework for Necessity-Based Self-Defense
“Why, it’s a Looking-glass book, of course! And if I hold it up to a glass, the words will all go the right way again.” — Lewis Carroll, Through the Looking-Glass (1897)
On January 3, 2020, the United States killed Iranian Major General Qassim Soleimani near Baghdad International Airport. The legal debate that followed exposed a persistent confusion in how we think about self-defense under international law, and incorrectly boiled it down to a single word — imminence. I argued in a Southwestern Journal of International Law symposium volume, and continue to argue, that imminence is the wrong lens. The right lens is necessity.
Jus ad bellum inquiries continue to reduce claims of self-defense to a single temporal question: was an attack imminent? The answer to that question rarely captures the full legal picture. In the years since the Soleimani debate, the same flawed imminence-centric analysis has resurfaced across a range of contemporary conflicts. Whether one agrees with that justification or not, it illustrates with stark clarity why the law needs a more coherent framework.
That framework exists; it simply requires an appropriate looking glass. Indeed, this very framework was alluded to as the legal basis for at least one recent military campaign in which a State argued it faced a closing window of opportunity to act before an adversary engineered a condition of strategic immunity.
The argument of this piece is straightforward: the Caroline standard, properly understood, provides a necessity-based analytical framework that is both legally sound and practically workable across the full range of contemporary conflicts. That standard does not require States to absorb the first blow. It does not license adventurism. And it does not leave the law at the mercy of whatever a State chooses to call “imminent.” What it demands is a rigorous, three-part showing that has real disciplining force and that can provide the public legal justifications that the international community is owed whenever force is used.
Over-Reliance on Imminence
Before turning to that framework, it is worth examining why the debate has stalled on imminence in the first place and why that framing is dangerous.
Article 51 of the UN Charter preserves the “inherent right of individual or collective self-defense if an armed attack occurs.” Critics of pre-attack uses of force routinely focus on the absence of an imminent attack, arguing that neither an adversary’s weapons development nor its pattern of aggression crosses the threshold required to trigger the right of self-defense. This is the traditional view—that self-defense requires an imminent threat—and it has permeated legal and political debate since the 1837 Caroline incident.
The problem is that imminence, elevated to the level of a necessary pre-condition, is both over- and under-inclusive as a limiting principle. As the late Professor Yoram Dinstein observed, “imminence may mean different things to different people: either too little or too much.” One can easily imagine an imminent threat that does not necessitate an immediate response, a border skirmish that poses no real threat to national security, for example. Conversely, there are cases where immediate military action may be warranted in response to threats that are not yet “imminent” in the classic sense but that will soon become impossible to address. The window closes; the moment of “last resort” arrives before the attack is formally imminent.
This dynamic is precisely what the critics miss. A focus on imminence alone risks precluding uses of force that are necessary from a national security standpoint, particularly where a State is engineering a condition of strategic immunity that will foreclose future options. Imminence is relevant to necessity, but it is neither equivalent to it nor a substitute for it.
The Caroline Standard, Properly Understood
The legal framework governing self-defense has its roots not in the UN Charter, but in an 1837 diplomatic exchange arising from a border incident near Niagara Falls along the U.S.-Canada border. After British forces destroyed the American ship Caroline, U.S. Secretary of State Daniel Webster articulated what has since become the foundational customary law standard for self-defense: a State must show a necessity of self-defense “instant, overwhelming, leaving no choice of means, and no moment of deliberation,” and the response must be neither unreasonable nor excessive, “limited by that necessity, and kept clearly within it.”
Webster’s formulation is misread to conflate necessity with imminence. Imminence is a relevant factor, but it is one consideration within a broader necessity analysis. The true Caroline test requires three conditions precedent to the use of force: 1) a State or actor must threaten or commit an armed attack or measures tantamount to one; 2) alternatives to force must have failed or be impracticable; and 3) competent intelligence must indicate that an armed response is presently required to protect national security interests.
This framework is capacious enough to address contemporary threats without collapsing into lawlessness. The remainder of this post examines what each condition demands in practice.
Applying the Caroline Conditions
Armed Attack or Measures Tantamount
The threshold question under Caroline is not whether a formal declaration of war has been made or whether bombs have fallen. It is whether a State or non-State actor has threatened or committed acts that rise to the level of an armed attack, or measures tantamount to one. This formulation, drawn from the International Court of Justice’s Paramilitary Activities judgment, is broad enough to capture the range of means through which modern adversaries employ force, including irregular proxy campaigns, maritime aggression, and cyberattacks with kinetic effects.
In practice, the first condition is rarely satisfied by a single discrete event. More often, the factual predicate is built from a pattern: sustained support for proxy networks that attack a State’s forces and allies; direct attacks on military installations or naval assets; and the systematic development of weapons systems whose stated or known purpose is to deter response and foreclose future action. Where an adversary has spent years constructing this kind of cumulative campaign of armed aggression—through irregular means, through proxies, and through its own forces—the “measures tantamount to an armed attack” threshold is not a difficult one to clear.
The weapons development dimension presents the most legally challenging questions. Under what Professor Dinstein called interceptive self-defense, a State may lawfully use force to counter an armed attack that an adversary has “committed itself to in an ostensibly irrevocable way.” The question is not who fired first, but who crossed the legal Rubicon. Where a State is actively building a weapons arsenal of such destructive potential—whether nuclear, chemical, or otherwise—that its use would pose an existential threat, and where that State has both the intent and the capability to deploy those weapons, that trajectory itself begins to satisfy the first condition. The adversary has, in effect, committed to a course of action. The question for the responding State is whether the window to act remains open.
Alternatives Failed or Are Impracticable
The Caroline standard does not require that every conceivable diplomatic avenue be exhausted to the point of futility. It requires that alternatives be impracticable, not merely incomplete. This is an important distinction. A State is not obligated to continue negotiating indefinitely with an adversary that does not honor its obligations or negotiates in bad faith or to delay.
The paradigm for this condition involves stalled diplomacy: talks that produce frameworks, commitments, and agreements, each subsequently abandoned or violated. Each cycle of engagement followed by defection is significant. It is evidence—cumulative, documented, and publicly available—that alternatives to force are not merely incomplete but genuinely impracticable. A determination of impracticability in such circumstances is not a rationalization for action that was predetermined; it is the logical conclusion of a documented record.
This condition also requires consideration of timing. Critics will often point to the proximity of military action to the most recent diplomatic exchange and argue that talks were abandoned prematurely. But the Caroline standard does not turn on the calendar distance between the last round of talks and the first strike. It turns on whether, given the totality of the record, there was a realistic prospect that further diplomacy would succeed. The proximity of a particular negotiating moment, or any other measure short of the use of force, to the use of force does not, by itself, establish that alternatives remained practicable.
Competent and Specific Intelligence
This is often the most contested element of any pre-attack use of force. The Caroline standard demands more than a generalized threat or a long-run strategic concern. It requires that competent intelligence—not wishful thinking, not political preference—indicate that action is presently required to protect national security interests.
In practice, this condition operates on two levels. The narrower level is tactical: is there intelligence indicating an offensive action is being planned or prepared in the near term? Where such intelligence exists and is reliable, the necessity of response may be clearest, provided the anticipated offensive action is sufficient to warrant an armed response.
The broader level is strategic: even absent specific tactical intelligence of an imminent strike, is there a “last chance” scenario in which the opportunity to prevent a catastrophically worse future will be permanently foreclosed? Professor Henry Shue’s “last resort” theory applies here. Consider a State that is producing weapons-grade material or conventional weapons at a rate that, within a defined window, will allow it to achieve a condition of strategic immunity, a point at which the cost of military action would become prohibitive and deterrence would shift catastrophically. Even if an attack is not imminent, immediate action may be necessary because delay eliminates the option entirely. That is the logic of preventive self-defense, and it has genuine legal weight under a properly understood Caroline analysis, provided the underlying intelligence assessment is competent and the factual record supports it.
Why This Matters
The Caroline standard, when properly applied, does not make necessity an empty vessel. It disciplines legal analysis by demanding a factual record that satisfies all three conditions. States cannot simply invoke “necessity” as a shibboleth. But neither can critics reduce the law to an imminence requirement that would force any nation to absorb a potentially catastrophic first strike from an adversary before responding.
What the framework demands of States is transparency. When a regime uses force under a necessity justification, it owes its own people and the international community a thorough public explanation, one that lays out the factual record on each of the three conditions, acknowledges the intelligence on which it relied, and explains why alternatives were impracticable. As Harold Koh has written, the goal is not simply legal compliance, but legitimacy: enabling others “to assure themselves that the government action is indeed justified under international law.” That duty of explanation is not a weakness. It is what distinguishes a lawful use of force from an act of aggression dressed in legal language.
The Caroline standard provides exactly the right framework for that explanation, one grounded in customary international law, consistent with Article 51 of the UN Charter, and wide enough to address the full range of threats that contemporary States face. It is time to hold the looking glass up to the words and make them go the right way again.
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Thomas W. Oakley is a retired Judge Advocate, former Assistant Professor of Law at the United States Military Academy, and a current staff member at West Point.
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: U.S. Navy, Petty Officer 3rd Class Damian Cook
