Deterrence and Triangular Coercion: The Limits of Third-Party Constraint

Deterrence and Triangular Coercion: The Limits of Third-Party Constraint

Deterrence is a strategy designed to prevent a challenger from continuing to attack in order to alter an existing status quo. In its classical formulation, actor A—the defender—seeks to deter actor B by issuing credible threats of the form “if you do X, I will do Y,” thereby raising the expected costs of action to the point where the prize is no longer worth pursuing. This basic logic generated an extensive literature examining the assumptions underlying deterrence theory, particularly the conditions under which threats are believed, how credibility is established, and why weaker challengers nevertheless continue to test stronger defenders.

One of the most persistent puzzles in this literature is the observation that weaker challengers often continue to initiate or sustain challenges against stronger defenders, and in some cases “win” by surviving long enough to fight another day. From the challenger’s perspective, survival itself may constitute a form of success, allowing continued resistance and repeated attempts to alter the status quo. This dynamic complicates traditional deterrence theory, which assumes that sufficient punishment or denial should eventually induce compliance.

The concept of cumulative deterrence—based on repeated acts of punishment and denial intended to degrade the challenger’s capabilities—has attempted to address this problem. The expectation is that over time, sustained pressure will exhaust the challenger’s capacity or willingness to continue contesting the status quo. However, this approach has left the causal mechanism under-specified: it is not clear precisely when or why a challenger should abandon its objectives, nor how repeated punishment translates into a decisive cognitive or strategic shift.

A more recent approach, articulated by Lieberman, emphasizes deterrence as a process of strategic learning. In this view, the decisive mechanism is not cumulative exhaustion per se, but the imposition of “strategic teaching” through military interaction that progressively eliminates or narrows the challenger’s available repertoire of strategies. As the set of viable military and political options shrinks, the challenger is forced to reassess its goals, and in many cases to abandon the struggle altogether. Deterrence stability thus emerges not simply from cost imposition, but from the systematic erosion of perceived pathways to success.

The classical deterrence framework was initially developed as a two-actor game in which actor A attempts to deter actor B. Success depends on the credible demonstration of both capability and resolve, often through actual or threatened war. Actions short of war are frequently interpreted as ambiguous, potentially allowing the challenger to discount them as bluff. In response to the limitations of this dyadic model, scholars expanded the framework to include indirect deterrence, introducing a third actor, C. In indirect deterrence, A, unable to influence B directly, applies pressure to C, which in turn has influence over B. However, this extension already suggests a credibility problem: reliance on C implies that A lacks sufficient direct leverage over B, revealing either a capability or resolve deficit.

More recent work on triangular coercion, articulated by Sobelman, offers a further refinement of this three-actor structure. In this formulation, it is not only the defender who seeks to deter the challenger, but the challenger who also seeks to deter the defender from engaging in effective deterrence or compellence. The weaker challenger attempts to prevent the defender from imposing the very conditions required for deterrence stability. As a result, the objective is not stable peace, but rather a condition of managed “no war, no peace,” in which the challenger can continue to apply pressure while constraining the defender’s ability to respond decisively. Temporary deterrence stability may emerge when the defender is confronted with unexpected or unacceptable costs associated with acting against the challenger’s indirect leverage.

This is the core of triangular deterrence. Unlike indirect deterrence, where A pressures C in order to influence B, triangular deterrence involves C threatening or exposing vulnerabilities in B that are strategically important to A. The challenger exploits this relationship to impose costs on the defender, thereby deterring the defender’s deterrence efforts. In this configuration, deterrence is not solely about preventing B from acting, but about preventing A from effectively enforcing deterrence in the first place.

Within this framework, triangular coercion depends on a chain of vulnerabilities: C must be able to influence B, and A must be sensitive to costs imposed through C. However, the effectiveness of this structure is not purely material; it is also psychological and political. It depends on whether A believes that harm to C will constrain its willingness to escalate. If that belief holds, C becomes a powerful conduit of coercive leverage.

Yet this logic also introduces a critical limitation. If actor A becomes willing to absorb or tolerate significant costs to C in order to restore long-term deterrence stability against B, the triangular structure begins to weaken. In such a scenario, A prioritizes strategic resolve over intermediary protection, signaling that third-party harm will not automatically constrain escalation. This shifts the system away from triangular coercion toward more direct forms of deterrence.

Under these conditions, coercive leverage through C loses effectiveness, because the intermediary can no longer reliably transmit pressure back onto A. What remains is a more direct confrontation between A and B, in which outcomes depend less on indirect vulnerability chains and more on relative resolve, capability, and willingness to sustain costs over time.

Ultimately, deterrence—whether direct, indirect, or triangular—depends on the interaction between material capability and perceived resolve. But the deeper mechanism that determines whether deterrence stabilizes is not simply the imposition of costs or the accumulation of punishment. It is whether the challenger comes to believe that its available strategies for success have been exhausted. When strategic options are systematically narrowed through sustained interaction, deterrence becomes stable. When they remain open—even at high cost—deterrence remains contested, and triangular coercion becomes a tool for managing rather than resolving conflict.

A contemporary illustration of triangular deterrence can be found in analyses of the 2026 U.S.–Iran conflict. Unable to defeat the United States or Israel directly, Iran reportedly sought to constrain American and Israeli military operations by threatening vulnerable Gulf states and disrupting maritime commerce through the Strait of Hormuz. By targeting energy infrastructure and creating the prospect of severe economic disruption for U.S. regional partners, Iran attempted to impose costs not directly on its military adversaries but on actors whose welfare mattered to them. This strategy produced what some observers described as “intra-war deterrence,” limiting escalation and helping preserve Iran’s ability to continue resisting despite its military inferiority. The case demonstrates how a weaker challenger can exploit third-party vulnerabilities to complicate a stronger defender’s deterrence and compellence efforts.

At the same time, the case highlights an important limitation of triangular deterrence. Iran’s leverage depended not simply on its ability to threaten Gulf states, but on the assumption that the United States would remain unwilling to absorb the political, economic, and strategic costs associated with harm to those states. The effectiveness of the strategy therefore rested on a particular assessment of American resolve rather than on the mere existence of third-party vulnerability. If the United States were willing to bear those costs in pursuit of a decisive military outcome and long-term deterrence stability, the coercive value of the Gulf states as intermediaries would diminish substantially. The case therefore illustrates that triangular deterrence may generate temporary constraints on a defender’s behavior without necessarily producing durable strategic deterrence. Thus, triangular deterrence can explain temporary restraint, but it does not by itself explain long-term deterrence stability, which depends on strategic learning and the challenger’s eventual conclusion that no viable path to success remains.