War between major states has been a focus of international relations (IR) inquiry in the West since the time of Thucydides. In the first half of the twentieth century, war presented a recurring problem – including two catastrophic world wars. The advent of nuclear weapons then made war a potential threat to the very existence of advanced industrialized civilization. The discipline of international relations was born between the two world wars in 1919 with the creation of the first chair in international relations at Aberystwyth (Porter Reference Porter1972: ix). Since then, scholars have learned much about the causes of war and how to avoid it. Nevertheless, much still remains to be learned. We have not found definitive answers either to the question of what causes war or to the question of how leaders can manage crises so as to avoid war.

Our book addresses these deficiencies by investigating the factors that distinguish crises that escalate to war from those that do not. This query has, since Bremer (Reference Bremer1992), guided much of the quantitative analysis of peace science. We, however, adopt a case-based approach, in which we compare seven historical crises that escalated to war with eight crises that did not. Each case considers systemic factors, the foreign policy interactions of states, and internal factors, with a focus on crisis decision-making. Through such an analysis, we aim to see what lessons we can derive from the included crises – in particular, what these crises tell us about the diplomacy of war and peace, as well as international relations theory more broadly. We ask what decisions leaders made in each crisis, why they took the actions they did, and how these decisions and actions led them closer to or away from war.

This study builds on James Richardson’s (Reference Richardson1994) Crisis Diplomacy: The Great Powers since the Mid-Nineteenth Century. As Richardson did, we only examine major-state war. We therefore consider only cases that involve major states (i.e., the strongest states in the system) on opposing sides of the crisis – with an eye toward whether and how these major states avoid war with each other. We define a crisis that “escalates to war” as one in which the major states fight a war against one another. Not all major-state crises end in a war between major states. The latter often involve situations in which minor states fight a local war and major states may be diplomatically involved, but they do not go to war with one another. For example, in the 1839 Eastern Crisis, the Ottoman sultan fights his vassal – Muhammed Ali, pasha of Egypt. Britain and Austria attack Muhammed Ali. This leads France to threaten war, but it ultimately decides against doing so. When, as here, the major states involved in a crisis do not go to war with one another – even if a major state fights a minor state – we consider the crisis as one that “does not escalate to major-state war.” We then analyze how and why the local war did not expand into a war between major states.

Despite this similar research design decision, we deviate from Richardson in an important respect. Unlike Richardson, we use the extensive statistical findings in international relations to understand our cases – particularly the factors known to affect the probability that a crisis escalates to war or settles peacefully (see Mitchell and Vasquez Reference Mitchell and Vasquez2025). This allows us to incorporate certain hypothesized generalizations to see both if these factors shape the interactions among our crisis actors and whether the case fits a well-understood pattern. Our in-depth analyses of each historical case tell us whether and how these statistical factors translate into actual decisions. This, in turn, permits us to evaluate whether related theoretical notions hold or prove anomalous in each case. Any anomalies often become a subject of discussion in the “Theoretical Analysis” section of each case-based chapter. Thus, although we follow Richardson’s emphasis on case studies as a way to address key questions, we deviate from his theoretical focus on bargaining to explain why crises escalate to war (see J. Richardson Reference Richardson1994: ch. 13).

Our limited number of cases prohibits us from systematically testing theories in terms of a “logic of confirmation.” Our inquiry is therefore better justified in terms of a “logic of discovery” (see Freyberg-Inan et al. Reference Freyberg-Inan, Harrison, James, Freyberg-Inan, Harrison and James2016). The latter requires what Clarke and Primo (Reference Clarke and Primo2012: 92) classify as an “exploratory model,” in which the researcher operates “with an eye toward explaining the events surrounding a specific case or small number of cases, or to ask and answer counterfactual or ‘what if’ questions regarding events such as the Cuban missile crisis.” Whether one can interpret the results of our empirical investigation as “discovering” a possible pattern that generalizes to other cases is an open question and not predetermined theoretically. We simply expect that our fifteen cases will help us “discover” possible factors that distinguish the crises that go to war from those that do not. In Chapter 16, we aggregate our within-case knowledge across cases (Goertz Reference Goertz2017). This yields some generalizable patterns, but these must be considered hypotheses that scholars will need to test systematically in the future using the logic of confirmation.

Our focus on decision-making has empirical advantages – chiefly, that we can directly observe what leaders and members of the various polities actually did and, in some cases, thought. It also gives us a certain theoretical leverage. For each crisis that escalates to war, we can use counterfactual reasoning to determine if (and how) war could have been avoided, as well as to understand why leaders were unable to avoid it. Similarly, for each crisis that does not escalate to war, we can consider the conditions under which the crisis might have ended in a war – and why it did not when it could have done so. By addressing such questions, our project aims to learn from actual historical cases about why and when war between major states occurs and how diplomacy might avoid it.

Our main research question consequently involves a search for generalizations; yet, we hope to do other things with our cases as well. We examine each case to see not only whether it might illustrate possible patterns that repeat but also whether it illuminates the particular factors that pushed the crisis toward or away from war. We are, in other words, interested in deriving lessons for future diplomacy from leaders’ decision-making in each individual case. As we pursue this task, we strive to keep an open mind about what the evidence in each case might be trying to tell us. We therefore do not deductively develop a list of possible lessons and then search for them within our cases; instead, we proceed inductively to see what lessons “emerge.”Footnote ¹ The lessons we derive range from “cautionary warnings” and mistakes that leaders should avoid to successful behavior that leaders might emulate if their goal is to avoid war.Footnote ² Through these lessons, we also explore whether the cases vindicate certain, accepted theoretical notions within the IR discipline – or alternatively, whether and why a case proves anomalous in light of such notions. These various lessons appear in the “Lessons” section of each case-based chapter; we justify our derivation of these lessons in the “Methodological Appendix.”

The seven cases in our study that escalate to a major-state war are: the Crimean War, the Italian War of Independence of 1859–1860, the Franco-Prussian War, the Russo-Japanese War, July 1914, Danzig 1939, and Pearl Harbor. The eight cases that do not result in a major-state war are: the 1839 Eastern Crisis, the Second Moroccan Crisis, the 1908–1909 Bosnian Crisis, Munich, the Berlin crises, the Cuban Missile Crisis, the Cod Wars, and the 2014 Crimea seizure and its related Russia–Ukraine War. In the latter, the United States and its allies in the North Atlantic Treaty Organization (NATO) did not intervene to fight militarily against Putin, so it is not considered a war between major states.

For each case, we explore why the decisions that leaders took led to or avoided war. We then explore historically and theoretically why they made those decisions. We typically begin each case analysis with a “Background” section that places the crisis within a wider context. The “Historical Events” section then reviews the major interactions within the case that made it a crisis and shaped whether it went to or avoided war. The most significant events appear in a “Nutshell Summary.” Next, a “Key Variables” section reviews the prominent factors that research suggests should distinguish the crises that go to war from those that do not – including their relative potency. This systematizes the analysis across cases. A “Theoretical Analysis” section then identifies the major theoretical questions that the case presents. Finally, each case study concludes with a set of “Lessons” for diplomacy, as well as broader implications for IR theory.

Five major parts comprise this work. Part I contains the foundations – namely, this Introduction and Chapter 1, which presents the book’s theoretical framework and research design. Parts II, III, and IV present our crisis cases individually in three groups – those from the post-Napoleonic nineteenth century, those from the twentieth century through World War II, and those from what we call the nuclear era. To explore the role of nuclear weapons in greater detail, this latter section contains four crises: two that directly involve nuclear weapons and two that do not. Finally, Part V offers a conclusion, consisting of two chapters that delineate the major lessons for world politics, in terms of the patterns present, what each crisis tells us, and general lessons for diplomacy and IR theory. To reach these conclusions, we compare our analyses and lessons across cases and integrate the insights gleaned into a set of overarching general lessons for world politics. We end with a brief discussion of the book’s implications for peace and the use of diplomacy to avoid war.