One of the central fields of study in international relations focuses on the process by which states intervene within the boundaries of other sovereign polities. Patrick Milton's monograph shows that the theory and practice surrounding intervention, which scholars have long conceptualized to postdate the French Revolution, instead have deep roots in the early modern period. Milton examines these issues through the lens of the Holy Roman Empire, whose decentralized, polycentric structure makes it well-suited for understanding the logic and operation of intervention. His argument has two main prongs. First, Milton uses intervention as a way to reassess the norms and operational principles of the empire's judicial structure. Second, he aims at the venerable Westphalian myth, which posits that the 1648 Peace of Westphalia granted the individual states of the empire full sovereignty and enshrined the idea of nonintervention in other states. For Milton, rather than making intervention within the empire less likely, the Peace of Westphalia actually increased opportunities for intervention by creating new mechanisms and justifications for both individual states and the imperial organs of government to intervene within the boundaries of imperial estates.

Milton unfolds his argument over seven body chapters. Chapter 1 examines the legal framework and institutional structures that facilitated intervention within the empire. He identifies two broad categories of intervention: regular, which happened under the authority of the emperor; and irregular, which occurred outside the emperor's authority and approval. Both forms of intervention drew on legal principles undergirding the imperial constitution and occurred in the name of protecting the safety of a territory and its subject from what contemporaries labeled “bad governance.” Milton breaks the final six chapters into two thematic sections: one focused on European interventions within the empire, and the other on interventions within the empire organized by the imperial estates. Each section is structured roughly chronologically stretching back to 1500, although the majority of the book focuses on the seventeenth and eighteenth centuries.

In the first thematic section on external inventions by foreign powers in the empire, Milton frames the Thirty Years’ War as the result of a series of interventions that prolonged and escalated the war at every turn. These interventions were framed as acts of protection against tyrannical lords and justified by claims that not intervening would threaten the security of the intervening party. This logic enabled interveners like Sweden and France to justify their endeavors on the basis of self-defense. When the war finally drew to a close with the Peace of Westphalia, the Peace's structure actually increased the likelihood of intervention in the empire by outside powers. This dynamic resulted primarily from the Peace's Guarantee Clause, which made numerous states, including France and Sweden, responsible for guaranteeing the observation of all aspects of the peace treaty. After Westphalia, therefore, these states had a ready-made excuse for intervening in the empire's territories in order to enforce the Peace's stipulations, as “the external guarantee tied the constitutional law of the Empire to the law of nations, by creating an international responsibility to uphold the Imperial constitution” (132). Ultimately, therefore, the Peace of Westphalia did not establish a balanced system of competing sovereign states but instead “helped to establish the principle of internationally guaranteed minority rights, enforceable through intervention, as part of the positive law of nations/international law” (155).

Part two of Milton's book turns to intervention within the empire's territories by other members of the empire. These internal interventions usually took the form of judicial executions, meaning imperial estates intervened within the borders of other estates in order to carry out legal orders issued by the empire's judicial bodies. Such interventions occurred most frequently in small territories. While they could be armed, more frequently they involved the appointment of a commission that instituted changes within the affected territory. After surveying the process of intervention in the empire's smallest states, Milton offers lengthy case studies of two eighteenth-century interventions in larger territories: Nassau-Siegen and Mecklenburg-Schwerin. In both cases, the process of intervention dragged on for years and involved a myriad of actors at all levels of the empire. According to Milton, while the various parties involved in interventions had different end goals, “the underlying normative emphasis on the limitations of princely powers and the unacceptability of arbitrary violence against subjects and fellow princes, and the belief that infractions of these norms called for intervention, was the same” (222). Milton concludes that while most of the norms that shaped intervention remained consistent from the sixteenth through the eighteenth centuries, the outcomes of interventions remained unpredictable, and their success inconsistent.

Overall, Milton's arguments about the ramifications of Westphalia, and in particular about the importance of the Guarantee Clause, are persuasive and mesh well with recent research on the empire. His effort to show that the modern regime of state intervention has its roots in the early modern period is also successful. One hopes that Milton's book will be read not just by historians but also by political scientists, as this monograph has much to offer scholars working in a wide range of fields.