Some countries exhibit consistency in their foreign policies toward allies and adversaries. Others swing between conflictual and cooperative behavior in unpredictable ways, leaving onlookers befuddled. Eleonora Mattiacci’s Volatile States in International Politics grapples with the puzzling and understudied phenomenon of volatile foreign policy behavior. Most IR scholars concern themselves with central tendencies, often dismissing volatility as mere noise. Mattiacci astutely recasts these perturbations as substantively informative. Combining theoretical precision and methodological sophistication, the book advances a convincing argument on the nature and origins of seemingly random inconsistencies in foreign policy actions. This timely contribution offers new insights into recent whipsawing foreign policy changes—changes that have the potential to infuse international politics with uncertainty and mistrust.

Before explaining the roots of volatility, the book carefully distinguishes it from others forms of change that have already garnered scholarly attention. These include unidirectional shifts such as escalation and rapprochement, as well as repeating cycles of behavior. Volatility instead refers to deviations from the mean that remain after accounting for such trends.

Foreign policy volatility, the book theorizes, stems from the interaction of two variables: power and interests. Power, conceptualized at the international level, refers to relative material capabilities. More power opens the door to more possible foreign policy behaviors. Only those with sufficient means can execute highly conflictual acts or afford the risk of highly cooperative overtures. Power provides an opportunity for volatility, but opportunity alone is insufficient. Interests, conceptualized at the domestic level, capture the heterogeneity of preferences among groups within a country that simultaneously enjoy some ability to sway foreign policy. As preferences diverge among factions, the competing pressures they bring to bear produce foreign policy inconsistencies. As with power, however, interest clashes alone cannot produce volatility. Volatility emerges only when a country has both sufficient relative power and substantial interest heterogeneity.

Mattiacci uses directed dyads, such as India’s behavior toward Pakistan, to test the argument quantitatively and qualitatively. The quantitative measurement of volatility presents sizable challenges. Mattiacci meets them with sophisticated methods using event data that record directed conflictual and cooperative acts. A statistical procedure accounts for trends in the data to distinguish volatility from other patterns of change. Volatility is the variance of the residuals that remain after this procedure. With a dependent variable in hand, Mattiacci tests the argument on rivalrous dyads from 1948–92 that appeared in two common rivalry datasets. Analyses consistently support the argument. Neither power, measured as material capabilities, nor interest heterogeneity, measured via an index accounting for domestic cleavages and access to policy makers, alone affects volatility. Only in conjunction, when states have both the opportunity (power) and precipitating cause (divided domestic interests), do we observe spikes in volatility.

A case study of French foreign policy toward the United States from the mid-1950s to the mid-1960s provides welcome nuance to the empirical analysis and illustrates the theory’s relevance for allies, not just rivals. Using archival and secondary materials, the historical account documents the staggering numbering of French changes of heart regarding decolonization, the Suez Canal, and NATO. It is only in 1962 when De Gaulle sufficiently consolidates control over French foreign policy, thereby stemming the influence of competing domestic groups, that some (rather uncooperative) stability returns to French foreign policy toward the United States.

Several contributions distinguish this book from prominent existing perspectives. First, volatility is not reducible to leader idiosyncrasies or partisan turnover in government. It instead emerges from diverse groups simultaneously competing for influence within countries with ample power. Second, by delving into domestic politics, Mattiacci’s account relaxes the unitary actor assumption that underpins accounts of foreign policy “hedging” or that attribute volatility to cycles of tit-for-tat reciprocity. Even if reciprocity cycles occur, Mattiacci offers valuable insights into why these cycles begin.

As with any study exploring new terrain, Mattiacci’s scrutiny of volatility prompts several questions for future scholarship. Most notably, What are the consequences of foreign policy volatility? The book offers reasonable conjectures—heightened uncertainty, impaired expectation setting, little long-term planning, misinterpretation, mistrust, and even conflict escalation. But what follows from volatility is also an empirical question that remains unanswered. Volatile behavior could be problematic for the reasons Mattiacci speculates. Yet volatility may be an asset in circumstances when hedging and ambiguity are optimal; for example, when deterring an adversary while restraining an ally. Relatedly, how might the recipient of volatile acts respond? Based on the Franco-American case, it is easy to envision US frustration with French indecisiveness. Yet, US policy makers likely longed for such indecisiveness when it was replaced with steadfast obstinacy as France sought strengthened ties with the USSR, China, and Cuba. A volatile ally or rival might be preferable to a consistently unhelpful one.

Questions similarly emerge on the mechanisms by which high interest heterogeneity leads to foreign policy inconsistencies. Conceptually, Mattiacci posits that interest divergence is maximized when the major cleavages among a population are reinforcing rather than offsetting (e.g., urban–rural divides align with ethnic divides). Such stark divides seem likely to produce partisan sorting. Accordingly, circumstances in which a government would offer foreign policy “compensation” to both factions could be rare (p. 32). Instead, partisan splits could yield foreign policy oscillations that come with a change in this ruling party, but this differs from the book’s emphasis on simultaneous influence. Future work may further investigate the circumstances—for example, under a coalition government—during which foreign policy responds to multiple sides in a fractured domestic political landscape.

Moreover, the book sparks curiosity about precisely how domestic factions sway foreign policy actions. That they do so is eminently plausible, such as French oil companies’ noted desire to ensure smooth operations in the Suez. It would be fascinating to study the relative importance of interest group lobbying, bureaucratic capture, or the unspoken intuition of political leaders eager to satisfy constituents as levers by which domestic groups steer policy. Quantitative analyses could also explore this further. The index for interest heterogeneity combines measures of (1) institutional procedures for transferring power, (2) societal cleavages, and (3) proportional electoral systems. Disaggregating the index could help gauge whether one factor drives volatility more than the others.

Volatility presents genuinely complex conceptual and measurement challenges. As noted, great care went into generating a quantitative measure that is as good as one could hope for. At the same time, a measure based on residuals inherently makes it difficult to make substantive interpretations. The well-executed case and anecdotes sprinkled throughout the book thus serve a valuable role as concrete instances of volatility in action. Given their quality and explanatory usefulness, I would have welcomed even more. Beyond illustrating the real-world implications of volatility, these cases provide fertile ground for a nuanced evaluation of alternative explanations, such as the possibility that volatility represents mixed messages that are either strategically optimal to send or that emerge from bureaucratic confusion in which government agencies unknowingly work at cross-purposes.

Inevitable questions aside, Volatile States in International Politics marks a major step forward because it makes sense of foreign policies that we may previously have labeled nonsensical. By shifting attention from the mean to the variance, Mattiacci’s book urges readers to reconsider exactly what constitutes noise, as opposed to substance, in international politics. Such reconsideration yields a sizable payoff because it offers novel insights into foreign policies in flux.