Leaning on Legionnaires: Why Modern States Recruit Foreign Soldiers

Why do modern states recruit legionnaires—foreigners who are neither citizens nor subjects of the country whose military they serve? Rather than exclusively enlist citizens for soldiers, for the past two centuries states have mobilized legionnaires to help wage offensives, project power abroad, and suppress dissent. A supply-and-demand argument explains why states recruit these troops, framing the choice to mobilize legionnaires as a function of political factors that constrain the government's leeway to recruit domestically and its perceptions about the territorial threats it faces externally. A multimethod approach evaluates these claims, first by examining an original dataset of legionnaire recruitment from 1815 to 2020, then by employing congruence tests across World War II participants, and finally by conducting a detailed review of a hard test case for the argument—Nazi Germany. The prevalence of states’ recruitment of legionnaires calls for a reevaluation of existing narratives about the development of modern militaries and provides new insights into how states balance among the competing imperatives of identity, norms, and security. Legionnaire recruitment also underscores the need to recalibrate existing methods of calculating net assessments and preparing for strategic surprise. Far from being bound to a state's citizenry or borders, the theory and evidence show how governments use legionnaires to buttress their military power and to engineer rapid changes in the quality and quantity of the soldiers that they field.

When Do Ideological Enemies Ally?

Why is it that international ideological enemies—states governed by leaders engaged in deep disputes about preferred domestic institutions and values—are sometimes able to overcome their ideological differences and ally to counter shared threats, and sometimes they are not? Alliances among ideological enemies confronting a common foe are unlike coalitions among ideologically similar states facing comparable threats. Members of these alliances are perpetually torn by two sets of powerful contending forces. Shared material threats push these states together, while the effects of ideological differences pull them apart. To predict when ideological enemies are and are not likely to ally in the pursuit of common interests, it is necessary to know which of these contending forces is likely to dominate at a particular time. The values of two ideological variables beyond that of ideological enmity play the key role in determining outcomes: (1) states’ susceptibility to major domestic ideological changes and (2) the nature of the ideological differences among countries. Similar levels of ideological enmity and material threats will have vastly different effects on leaders’ alliance policies as the values of these additional ideological variables alter.

PRC Assertiveness in the South China Sea: Measuring Continuity and Change, 1970–2015

Why has the People's Republic of China (PRC) courted international opprobrium, alarmed its neighbors, and risked military conflict in pursuit of its claims over vast areas of the South China Sea? Answering this question depends on recognizing long-term patterns of continuity and change in the PRC's policy. A new typology of “assertive” state behaviors in maritime and territorial disputes, and original time-series events data covering the period from 1970 to 2015, shows that the key policy change—China's rapid administrative buildup and introduction of regular coercive behaviors—occurred in 2007, between two and five years earlier than most analysis has supposed. This finding disconfirms three common explanations for Beijing's assertive turn in maritime Asia: the Global Financial Crisis, domestic legitimacy issues, and the ascendancy of Xi Jinping. Focused qualitative case studies of four breakpoints identified in the data indicate that PRC policy shifts in 1973, 1987, and 1992 were largely opportunistic responses to favorable geopolitical circumstances. In contrast, the policy change observed from 2007 was a lagged effect of decisions taken in the 1990s to build specific capabilities designed to realize strategic objectives that emerged in the 1970s.

Wartime Commercial Policy and Trade between Enemies

Why do states trade with their enemies during war? States make deliberate choices when setting their wartime commercial policies, and tailoring policies to match the type of war the states are expecting to fight. Specifically, states seek to balance two goals–maximizing revenue from continued trade during the war and minimizing the ability of the opponent to benefit militarily from trade. As a result, states trade with the enemy in (1) products that their opponents take a long time to convert into military capability, and (2) products that are essential to the domestic economy. Furthermore, states revise their wartime commercial policies based on how well they are doing on the battlefield. An analysis of British wartime commercial policy in World War I finds that a product's conversion time into military capabilities determines if and when that product will be prohibited from trade during the war. Alternatively, domestic political pressures play only a marginal role in wartime commercial policy decisions.

Illusions of Autonomy: Why Europe Cannot Provide for Its Security If the United States Pulls Back

Europe's security landscape has changed dramatically in the past decade amid Russia's resurgence, mounting doubts about the long-term reliability of the U.S. security commitment, and Europe's growing aspiration for strategic autonomy. This changed security landscape raises an important counterfactual question: Could Europeans develop an autonomous defense capacity if the United States withdrew completely from Europe? The answer to this question has major implications for a range of policy issues and for the ongoing U.S. grand strategy debate in light of the prominent argument by U.S. “restraint” scholars that Europe can easily defend itself. Addressing this question requires an examination of the historical evolution as well as the current and likely future state of European interests and defense capacity. It shows that any European effort to achieve strategic autonomy would be fundamentally hampered by two mutually reinforcing constraints: “strategic cacophony,” namely profound, continent-wide divergences across all domains of national defense policies—most notably, threat perceptions; and severe military capacity shortfalls that would be very costly and time-consuming to close. As a result, Europeans are highly unlikely to develop an autonomous defense capacity anytime soon, even if the United States were to fully withdraw from the continent.

Conventional Counterforce Dilemmas: South Korea's Deterrence Strategy and Stability on the Korean Peninsula

In response to North Korea's nuclear weapons program, South Korea is quietly pursuing an independent conventional counterforce and countervalue strategy. This strategy is unique. Few, if any, nonnuclear states have sought to rely on advanced conventional capabilities to deter a nuclear-armed adversary. Why is South Korea pursuing a conventional counterforce and countervalue strategy, and what could its impact be on strategic stability on the Korean Peninsula? South Korea's approach should be understood as both a short- and long-term hedge against U.S. abandonment. Its deterrent effect, no matter how uncertain, acts as a short-term stopgap if the United States abandons South Korea. Over the long term, capabilities such as advanced ballistic and cruise missiles bolster South Korea's nuclear latency. At the same time, we highlight that the strategy poses numerous technological and operational difficulties and has negative implications for arms race and crisis stability. Given South Korea's approach and North Korea's response, disarmament efforts focused purely on the bilateral U.S.–North Korea relationship will not succeed. Rather, any agreement will now need to address the growing gap in the conventional balance of forces on the Korean Peninsula.

Water and Warfare: The Evolution and Operation of the Water Taboo

For much of human history, water was a standard weapon of war. In the post–World War II period, however, nation-states in international conflict have made concerted efforts to restrain the weaponization of water. Distinct from realist and rationalist explanations, the historical record reveals that water has come to be governed by a set of intersubjective standards of behavior that denounce water's involvement in conflict as morally taboo. How did this water taboo develop, and how does it matter for nation-states? Focused process-tracing illuminates the taboo's development from the 1950s to the 2010s, and indicates that (1) a moral aversion to using water as a weapon exists; (2) this aversion developed through cumulative mechanisms of taboo evolution over the past seventy years; and (3) the taboo influences states at both an instrumental level of compliance, and, in recent decades, a more internalized level. These findings offer new avenues for research and policy to better understand and uphold this taboo into the future.

The Rule of Law and the Role of Strategy in U.S. Nuclear Doctrine

In 2013, the U.S. government announced that its nuclear war plans would be “consistent with the fundamental principles of the Law of Armed Conflict” and would “apply the principles of distinction and proportionality and seek to minimize collateral damage to civilian populations and civilian objects.” If properly applied, these legal principles can have a profound impact on U.S. nuclear doctrine. The prohibition against targeting civilians means that “countervalue” targeting and “minimum deterrence” strategies are illegal. The principle of distinction and the impermissibility of reprisal against civilians make it illegal for the United States, contrary to what is implied in the 2018 Nuclear Posture Review, to intentionally target civilians even in reprisal for a strike against U.S. or allied civilians. The principle of proportionality permits some, but not all, potential U.S. counterforce nuclear attacks against military targets. The precautionary principle means that the United States must use conventional weapons or the lowest-yield nuclear weapons that would be effective against legitimate military targets. The law of armed conflict also restricts targeting of an enemy's leadership to officials in the military chain of command or directly participating in hostilities, meaning that broad targeting to destroy an enemy's entire political leadership is unlawful.