Countless hours and pages have been devoted to solving what John Vasquez famously called "the war puzzle"—why do nations go to war?[1] Given the complexity and diversity of war, no single cause will ever be discovered. Nevertheless, discerning links between certain factors may help us explain and even predict its onset.

In Economic Interdependence and War, political scientist Dale Copeland (Univ. of Virginia) asks whether "economic interdependence between great powers … [has] a significant effect on the probability of war between them, and if so, does it decrease or increase the likelihood of conflict?" (1). This is a timely question, given the robust interrelations among global trading partners and recent destabilizing moves by the United States.

Relying on both quantitative and qualitative research, Copeland explores what he terms "trade expectations theory," which centers on a key variable: "a state's expectations of the future trade and investment environment." The theory posits that leaders of states dependent on trade who have positive expectations of future trade tend to prefer the benefits of peace over the costs of war. If, however, those expectations are negative or seem to portend steep economic decline, they "will begin to view war as the rational lesser of two evils" (2), fearing greater future vulnerability to attack or other coercion more than present armed conflict. In short, trade expectations theory combines "liberal theory's emphasis on the benefits from trade and investment with realism's concern for the potentially significant costs of adjustment that a state would face were it to be cut off after becoming dependent on such commerce" (429).

Copeland's quantitative analysis shows that, in thirty of his forty case studies, spanning the period 1790–1991, "economic interdependence played a moderate to strong causal role in shaping the events" (93), and that trade expectations theory is validated in twenty-six of those thirty cases. His qualitative inquiry extends over six chapters on (a) the Russo-Japanese War and German wars for hegemony (1890–1939); (b) Japan's machinations in the decades before Pearl Harbor (1905–40) and (c) considerations of Russia and the onset of World War II in the Pacific (1941); (d) the origins, dynamics, and termination of the Cold War (1942–91); (e) European power politics (1790–1854); and (f) European great power colonial rivalries (1856–99). Copeland believes these studies confirm "that it is indeed the expectations variable that is doing the causal heavy lifting in the vast majority of case periods where economic interdependence is implicated in the outbreak of war or peace" (433).

In regard to this apparent influence of trade expectations on the likelihood of war, Copeland introduces the "trade-security dilemma, [which] involves the implications of actions that states take to improve the certainty of future access to resources, investments, and markets" (10). Actions taken by state A to secure its trade and economic status may lead states B, C, and D to withhold trade lest state A gain too much power, become aggressive, and precipitate a "trade-security spiral" (11).

Finally, Copeland assesses the significance of his findings specifically for economic relations between the United States and China. He is optimistic that, so long as the Americans support open trade, the Chinese will eschew economic or other provocations. Today, however, President Donald Trump's administration is jeopardizing the fluidity of commerce between the two countries. Hence Copeland's analysis is acutely relevant to present-day conditions that may provoke military conflict.

It must be noted that Copeland in fact puts forth no purely quantitative analyses. He proceeds by determining possible causes of war in each of his case studies in a subjective manner, producing debatable findings that have, in some cases, been actively contested in the scholarly literature. His conclusions about casūs belli are opinions, not empirical results.

Taken as a whole, however, the book is an engrossing investigation of a critical variable in international relations vis-à-vis decisions to go to war. It elucidates specific foreign policy decisions by identifying and evaluating unit-level variables while preserving the realist stress on the distribution of power within the international order. Copeland's argument is particularly cogent and well-sourced regarding World War II-era Japan's preoccupation with China and Southeast Asia rather than the USSR, which the author argues was its initially preferred target.

Readers seeking an engaging historical examination of war and international relations theory will learn much from Dale Copeland's Economic Interdependence and War, especially in light of its pertinence to current events.