Security Studies Program Seminar

Nothing to Fear but Fear Itself?  Nuclear Proliferation and Preventive War

Alexandre Debs and Nuno Monteiro
Yale University

November 3, 2010

Introduction and Questions
For the last forty years, the United States and others have sought to prevent the spread of nuclear weapons.  To do so, these states have offered potential proliferators positive inducements (such as peaceful nuclear energy assistance), but have also threatened punitive action.  In extremis, states looking to halt the spread of nuclear weapons have threatened to attack potential proliferators in a type of preventive conflict. 

This situation begs two related questions.  First, when does counterproliferation – that is, the prevention of nuclear weapons acquisition - succeed?  Second, when does it lead to preventive war? 

Existing Theories
The existing literature does a poor job addressing these questions.  Five distinct literatures apply, but all are wanting. The nuclear proliferation literature, for instance, anticipated increased proliferation with the end of the Cold War, but did not anticipate a world in which only one state unambiguously joined the ranks of the nuclear powers and few preventive strikes occurred.  By this measure, counterproliferation has apparently succeeded.  The second literature, deterrence theory, poorly addresses the post-Cold War world in which an imbalance of power exists between deterrers (e.g. the U.S.) and target states.  Under these conditions, it is unclear how deterrers can credibly commit to not penalize target states even if target states comply with the non-proliferation demands.  Balance of power theory, on the other hand, insufficiently recognizes the fact that nuclear proliferation is the result of an active state decision that other states can respond to in different ways – changes in the balance of power are not automatic.  A fourth literature discusses preventive war.  This literature focuses on how changes in the distribution of power between states of roughly equal strength can lead to conflict, but this concept has limited applicability – the universe is much larger and diverse.  The final camp, exemplified by Jim Fearon’s work, addresses rationalist explanations for war.  The rationalist literature highlights the tendency of commitment problems and imperfect information to lead to conflict, but assumes that large and rapid shifts in the balance of power happen exogenously, an assumption which is not appropriate in understanding nuclear proliferation and preventive war.

Drawing from elements of each existing literature, Monteiro and Debs argue that nuclear proliferation can be conceptualized as a state investment in a type of capability that is 1) costly, 2) not perfectly observable by other states, and 3) has delayed returns (i.e. requires a lengthy period to develop and operationally deploy). Counterproliferation thus becomes a deterrence problem in which 1) State B (the proliferator) is considering investing in a nuclear program, and 2) State A (the deterrer) is trying to avoid this and can wage preventive war to stop it.  Three variables then affect the stability of deterrence.  The first is the effect of nuclear acquisition on the balance of power from State A’s perspective: what will State B’s acquisition do vis-à-vis A’s interests, and what are A’s capabilities to offset B’s potential gains?  The second is the cost of preventive war: how costly would it be for A to undertake preventive action against B to prevent nuclear acquisition?  The final variable is the quality of information available to a deterrer (A) about a target’s (B) decision: how confident is A that B is developing a nuclear program, and how readily can this information be confirmed or disconfirmed? 

These three variables interact to predict both when we expect counterproliferation to be successful and deterrers to resort to preventive conflict.  Simply put, Debs and Monteiro propose a rationalist theory juxtaposing the expected consequences of proliferation – what they term “nuclearization” – on the balance of power with the expected costs of preventive war prior to nuclearization.  The core prediction is simple: as the effect of nuclearization on the balance of power increases relative to the cost of preventive war, the probability of proliferation decreases.  As the shift in the balance of power grows relative to the cost of preventive war, State A will have a clearer interest in preventing B from acquiring nuclear weapons.  It will therefore be able to more credibly threaten B with preventive action to halt its nuclear program.  Assuming both states have perfect information, when the effect on the distribution of power is greater than the cost of war, then the threat of war should be perfectly credible, B should be deterred from acquisition, and thus no war will occur.  When, however, the effect on the distribution of power is less than the cost of war, then A’s threat will not be credible and proliferation will occur, though peace will still prevail (i.e. no war occurs)

States, however, rarely operate with perfect information - there is usually some large degree of uncertainty in international relations.  As a result, it is possible that war and proliferation by misperception can occur.  This is particularly an issue when the expected shift in the balance of power is greater than the expected cost of preventive war.  Here, imperfect information affects A’s confidence in the status of B’s nuclear program: because it is difficult to “prove a negative,” A may not be confident that B is not pursuing a clandestine nuclear program.  As a result, it is conceivable that A will launch a preventive war on B out of the mistaken belief that counterproliferation is failing even though B truly lacks a nuclearization program.  Table I summarizes the core predictions.

Table 1



Effect on Nuclearization vs. Cost of Preventive War



Perfect Information


Counter-proliferation fails and preventive conflict does not occur (same as H3)


Perfect Information


Counter-proliferation succeeds and preventive war does not occur


Imperfect Information


Counter-proliferation fails and preventive conflict does not occur (same as H1)


Imperfect Information


Counter-proliferation succeeds but preventive war may occur due to misperception**

Testing the Theory
How do these predictions play out in practice? Structurally, the Cold War world was a situation in which the effect of nuclear acquisition was low, particularly relatively to the cumulative assets of the superpowers, and prospective costs of preventive war high due to the threat of escalation.  This created extreme values on two of the independent variables.  The post-Cold War world, in contrast, has the opposite valuations as the effect of nuclear acquisition in an era of U.S. hegemony, is high while the costs of preventive conflict – particularly for the United States – is comparatively low.

At the grossest level, there were more proliferation efforts in the Cold War period than in the post-Cold War era.  Indeed, six to eight states had nuclear programs at any given point during the Cold War (1945-1989), while only two to three states had such programs afterwards.  As the theory would expect, it appears counterproliferation is more successful in an environment where the threat of preventive conflict is more credible than otherwise.

Additional evidence comes from particular deterrence episodes where a state debated whether to engage in preventive conflict to prevent nuclearization by an adversary.  Because of its future effect on the Cold War competition, a critical first case looks at U.S. responses to Soviet acquisition of nuclear weapons circa 1949.  Debs and Monteiro argue that the effect of Soviet nuclear acquisition on the balance of power was low.  The Soviets already had great conventional power that could be used to challenge the U.S.  As a result, Soviet nuclear weapons acquisition augured a moderate increase in capabilities rather than a step increase.  More importantly, however, U.S. leaders expected preventive war to be extremely costly.  Not only did policymakers believe the Soviets would overrun Western Europe in wartime, but the U.S. could not be confident it could impose a significant cost on the Soviet Union in return.  Indeed, the U.S. nuclear monopoly was rather weak at this time: there were few weapons, fewer delivery systems, and limited intelligence on the Soviet military establishment – we simply could not figure out what to bomb.  Ultimately, and as the theory would predict, the U.S. allowed its nuclear monopoly to expire peacefully as counter-proliferation failed but led to peace.

The Soviet case stands in contrast with the Iraq War of 2003.  The Iraq War is a case of mistaken preventive war.  Judging from public statements and what we know of internal deliberations, the Bush Administration saw dramatic effects of Iraqi nuclear acquisition on the balance of power, all of which operated to the detriment of the United States. For the U.S., though, the cost of preventive war was low: not only was Saddam’s military a hollow force after over a decade of sanctions following the 1991 war, but no state could credibly threaten to punish the U.S. for striking Iraq.  At the same time, though, the evidence of an Iraqi WMD program was limited.  While there was some evidence Iraq had such a program, equal evidence argued against Iraq being a potential proliferator.  Nevertheless, the U.S went to war at least part out of the concern that the U.S. could not verify that Iraq did not have a nuclear program, and thus it was better to be safe than sorry. 

Conclusion and Implications
If this argument is correct, what does it suggest about the future of preventive conflict for counter-proliferation purposes? It is interesting to think of this in the context of the Iranian nuclear program.  Both the United States and Israel worry about the effects of Iranian nuclearization.  The effects, however, are greater for Israel than for the United States.  On the other hand, the costs of a preventive conflict are lower for the United States than for Israel.  There is also poor information on the Iranian program for both states and a low tolerance for risk.  In light of the above framework, this suggests there is a large probability a preventive conflict of some kind will erupt.

Questions and Answers
The question and answer session focused on four general themes.  First, several participants wanted to know whether looking at the number of proliferation programs in existence at any given point in time is a reliable test of the theory.  Not only is there a data problem with identifying proliferation programs, but if one controls for nuclear regimes – such as the Non-Proliferation Treaty – the Cold War rate drops off precipitously.  Monteiro and Debs pointed out that the rate of proliferation remained steady between the adoption of the Non-Proliferation Treaty and the end of the Cold War and argued that the NPT was not convincing alternative interpretation for the drop at the end of the Cold War.

A second theme addressed the individual cases used to test and explore the theory.  In particular, participants questioned whether the Iraq War, given all that is now known of ulterior motivations and a confluence of circumstances, is an apt case of preventive war by misperception.  Debs and Monteiro maintained that Iraq was a mistaken preventive war but acknowledged the need to cover other proliferation episodes as the project moved forward.

A third line of questioning centered on whether the theory needed to account for the “expected effects of proliferation on the balance of power,” or whether this could be compressed into a story about the costs of preventive war relative to the costs of no action.  Indeed, if one focuses on the cost story alone, then it seems that a different prediction emerges: states will engage in preventive conflict for counter-proliferation purposes when the costs of the action are low, but will forego conflict if the costs are expected to be high.  Thus, the U.S. attacked Iraq – which is suspected of a nuclear program – rather than more powerful North Korea or Iran where the evidence of a program is much firmer.  Debs and Monteiro conceded that the effect of proliferation on the balance of power could be relabeled ‘the cost of inaction’, but argued that the simpler theory which only considered the cost of preventive war was not a sufficient explanation, as it missed an important strategic consideration.

A final theme focused on how states calculate the effects of nuclearization on the balance of power.  On the one hand, policymakers themselves may well offer hypotheses on the issue without any serious analysis – this makes the expectations more hyperbole than a serious calculation that could be used to measure the theory.  At the same time, it seems that the effects of nuclearization hinge in large part on the target state’s capacity to credibly reassure the deterrer about its future behavior; this opens up an entirely new line of inquiry.  Debs and Monteiro did not dispute these points.  Rather, they argued that understudied expectations of the post-nuclearization balance of power are acceptable indicators of their theory so long as decisions are made on the basis of these expectations.  Likewise, reassurance by the target state is important, but it is difficult for target states to reassure deterrers.  This is one of the reasons the Iran situation is so precarious: with a history of U.S.-Iranian animosity, there is little any foreseeable Iranian regime could do to assuage U.S. concerns about the Iranian nuclear program.

Rapporteur: Josh Shifrinson

back to Wednesday Seminar Series, Fall 2010