Rebecca Thorpe opens her book with a big question. How did the United States—constructed on a deep suspicion of professional militaries, foreign entanglements, and concentrated war powers—end up with the world’s most powerful armed forces, mounting defense budgets, and a habit of far-flung military interventions led by presidents of both parties? Thorpe’s route into this question is through a close examination of the changing role of Congress in military policy since the nation’s founding. She draws on new and innovative data sources to expose shifts in the incentives and interests of legislators and the ways in which these have, in turn, altered the opportunities and constraints facing the executive. The American Warfare State makes a number of signal contributions.

Most importantly, Thorpe offers a persuasive and detailed analysis of why the United States has sustained such high levels of defense spending since World War II, despite numerous shifts in party control of government and the rise and fall of national security threats. For most of the country’s history, she points out, the size of U.S. forces and spending levels dropped sharply after major conflicts or changes in the threat environment. But the pattern ended with World War II. The war’s end did not trigger the expected reduction in spending, nor did the end of the Korean or Vietnam conflicts or the Cold War. What changed?

The author’s research provides rich and wide-ranging insights on the subject. Her central contribution lies in a claim about the political economy of defense spending. Thorpe shows that the unprecedented mobilization for World War II led to the emergence of major military industries beyond traditional metropolitan defense hubs. In every decade since, defense expenditures continued to spread to new communities. The result was a vast increase in the number of Americans whose jobs or local economies depend on defense dollars. She then makes a shrewd analytic intervention. She demonstrates that the impact of defense contracts falls unevenly on local economies, exerting an outsized effect on less populated, less economically diverse semirural and rural communities (p. 179). To assess the political impact of this development, Thorpe marshals evidence that members of Congress from these defense-reliant districts are more active on military issues, more supportive of continued high spending levels, and more reluctant to withdraw funding from military operations than are their colleagues, including those within their own party whose ideological inclinations they share. As the benefits of military spending grew and spread, she adds, the costs were deferred or displaced, in part through increased deficit spending (pushing costs onto future taxpayers) and the creation of an all-volunteer military (shifting the burden of service to a limited segment of the population).

In developing this argument, Thorpe’s book sheds new light on institutions and interbranch relations in the postwar era. The framers of the Constitution, she reminds us, explicitly divided military authority between Congress and the president: The legislature’s power to raise armies, declare war, and control funding was designed as an institutional constraint on executive ambitions. The author’s central insight, a second major contribution of the book, is that this system of checks and balances rests on a structure of incentives that has collapsed in the years since World War II.

Given the lack of a standing army (soldiers were provided by state militias) and military production infrastructure (weapons were largely bought from foreign manufacturers), and given the limits on federal fiscal capacity (due to a narrow tax base and limited ability to borrow), early presidents had to appeal to Congress for military resources. Because legislators had to extract these resources from constituents through increased taxes, the founders assumed that lawmakers would grant funds and authority only in extraordinary circumstances, in amounts that were absolutely necessary, and for as short a time as possible. This logic facilitated rapid demobilization and spending reductions in the aftermath of conflicts from the Revolutionary War through World War I.

Incentives began to shift in the nineteenth century, however, with the rise of domestic weapons production, the capacity of the federal government to cover costs by printing money or taking on debt, and a more professional standing army. But it took the creation of a permanent and geographically widespread military apparatus for World War II to decisively shift the underlying congressional logic, as local economic imperatives “created new legislative incentives to procure ongoing defense resources, rather than demobilizing as had occurred after previous wars” (p. 183). Thorpe’s critical analytic contribution is to show how this shift in incentives undercuts the legislature’s capacity to serve as a check on the executive. Legislators’ interests in preserving their constituents’ jobs not only leads to permanent and unnecessary levels of defense spending (independent of national security threats), she argues, but also erodes Congress’s capacity to exercise control over the military through budgetary constraints. Her analysis is thus at once a persuasive account of when and why institutional constraints may fail, and a detailed chronicle of an important historical shift in interbranch relations.

In addition to these substantive contributions, the book provides a first-rate example of effective and well-designed mixed-methods research. It includes extensive original research, including a data set of locations of major defense industries in every state and congressional district, and a tracking of defense subcontract expenditures by congressional district, along with county-level estimations of local economic diversity. Thorpe presents evidence of the effects of defense reliance on the committee-assignment choices and voting records of members of Congress, on issues from defense spending and arms sales (in the 1990s) to authorization and continued funding for the war in Iraq (in the 2000s). On the qualitative side, the book reflects close readings of historical sources and early debates over the Constitution, as well as well-crafted historical-institutional arguments addressing, for example, the respective roles and mutual impacts of economic factors and government policies in the expansion of America’s military production capacities. The use and integration of this range of research methods in the service of a clear and carefully developed argument is a major accomplishment of the book.

The American Warfare State, in short, is impressive in both the scope and depth of its contributions. However, it also raises a number of issues and questions that merit further consideration and conversation among scholars of military and congressional politics.

First, the book goes some distance—but could elaborate further—on the question of where and how the incentive structure driving defense spending intersects with other explanations for military policy, particularly those focusing on the role of the executive rather than Congress. At several points, Thorpe notes that “perpetual military mobilization skews the institutional playing field heavily in favor of the executive” (p. 137), and facilitates but does not determine the increased use of force abroad. In a few places, however, she seems to subsume a range of factors driving executive military action under her core explanation for increased defense spending, arguing, for example, that “[h]eightened executive ambition, legislative atrophy, and expansive interpretations of the president’s constitutional war powers are all symptomatic of a new underlying incentive structure” (p. 180). These factors have multiple sources, some tied (but not reducible) to changed congressional incentives, others not.

The argument is most persuasive when the author acknowledges that increased opportunities and reduced constraints do not in themselves determine or explain whether or how those capacities are used or toward what ends, and explores interactions with the fuller range of factors that inform executive (and congressional) decisions (p. 131). The brief mention in her conclusion that the rising economic imperatives at the center of her analysis “coincided with the emerging dominance of a political ideology that views American military supremacy as a moral force for good” (p. 180) is an example of an insight ripe for fuller development and integration within her argument.

Finally, two of the work’s most innovative contributions—about economic reliance and institutional failure—raise intriguing issues for further scholarship and debate. Thorpe’s central insight about the political significance of (and variations in) local economic dependence on government spending is, at one level, a powerful explanatory argument about the scale and development of the nation’s military apparatus. But it is also presented as a “theory of economic reliance” (p. 23), and this poses two interesting questions. First, what are the limits of the theory in explaining defense spending trends, particularly in light of the fact that many members of Congress are not from defense-reliant districts? Second, how generalizable is it? What are the prospects for applying the concept of economic reliance to other policy arenas? Does it provide any leverage, for example, in understanding the politics of health care? This is another area of government spending that is vitally important in many communities, and in which local economic interests (as well as the interests of constituents) often run contrary to the positions of those members of Congress committed to limiting spending for ideological and partisan reasons.

The argument about institutional failure, likewise, has far-reaching implications. Thorpe’s research demonstrates that members of Congress often act according to a structure of individual interests and incentives defined by constituent and partisan pressures, in ways that undermine Congress’s institutional check on executive military action. Under what conditions, if any, will Congress defend its institutional prerogatives against the military ambitions of the executive? Can the institution function as more than the sum of its parts (i.e., the interests and incentives of its individual members)? What can we learn from the experience of the War Powers Resolution and subsequent congressional efforts to limit unpopular military operations? If, as Thorpe writes in the book’s final sentence, “[i]nstitutional checks and competing interests do not reliably limit power or promote the public good” (p. 185), where might we look for solutions? Are there institutional reforms that could restore the congressional constraint, or does the answer lie outside of the system of checks and balances?

Many of these questions reach beyond the scope of Thorpe’s project and are markers of success, not indicators of shortcomings, in the work. The American Warfare State does exactly what a groundbreaking book should do: It provides new evidence and analysis in a cogent argument, and leaves readers mulling new questions, provoked by its findings, about larger implications and future research agendas in American politics.