Patrick Milton set himself a difficult task in writing Intervention and State Sovereignty in Central Europe. At its core, the book is a study of foreign state interventions meant to protect subjects from unjust rulers in early modern Europe, particularly the Holy Roman Empire. The book's many case studies elaborate on the Empire's legal-constitutional processes permitting interventions, the normative basis that justified them, and the shifting political conditions that enabled them.

One might expect early modern interventions to stem primarily from geopolitical concerns, with rulers cynically taking advantage of the appeals of oppressed subjects to justify military action. While those examples exist, Milton's primary argument is that “the defence of other rulers’ subjects formed part of the motivation for interventions, while usually merging with, or serving as a pretext for, geopolitical motives” (5). Put another way, “Often a genuine concern for subjects’ rights was inseparable from a power-political appraisal of interests” (12-13). This argument, that concern for foreign subjects genuinely motivated interventions, is difficult to prove. Milton manages this problem well, showing that actors’ public justifications for interventions generally matched their private, internal correspondence, suggesting that the argument was more than mere lip service. For those seeking a more Realpolitik perspective, such as France's support for “German liberties” meant to weaken the Austrian Habsburgs, the book reminds that “rules and norms shape states’ perception of their self-interest” (12). Much like today, few interventions developed entirely from altruism or purely geopolitical factors.

The first chapter is the book's interpretive core, covering the Empire's legal-constitutional and normative frameworks for formal intervention, which remained largely the same throughout the period. The Empire's Eternal Peace of 1495 juridified political and social conflict by outlawing feuds. Rulers, estates, and subjects were expected to seek redress through the Reichshofrat, the Emperor's court, instead of resorting to the battlefield. The Reichshofrat's goal was mediation, but when that failed, intervention in the Emperor's name as highest judge and protector followed. Milton terms these top-down assertions of authority “vertical interventions.” The Empire's decentralized structure meant that regional powers, usually co-directors of the Kreise, carried out the actual interventions. Milton terms these, and other interventions between powers of roughly similar status, such as France and Austria, “horizontal interventions” (16).

Chapter 1 also explores the normative values rulers used to justify interventions, particularly those stemming from public law, natural law, and the law of nations. Scholars put forth a wealth of competing opinions on the ideal relationship between rulers and subjects and, important to this work, the conditions that could legitimate foreign intervention. Rulers and subjects thus both had ample material with which to appeal for support. As the following chapters show, subjects sued and requested interventions to stop maltreatment, maintain corporate rights, and restore the constitutional status quo. Rulers generally promoted the principle of nonintervention, seeing interference as an attack on their rights.

With that context, the rest of the book pursues a series of case studies. Part One (chapters 2-4) covers European interventions in Central Europe, which were “often a structural feature of the escalation of conflicts within polities” (56). Chapter 3 interestingly analyzes the Thirty Years’ War as a war of interventions, with the Elector Palatine, king of Denmark, king of Sweden, and king of France successively intervening (allegedly) to protect the rights and liberties of oppressed subjects. Despite these rulers’ power-political objectives, Milton convincingly argues that they “truly cared about German Liberties because they did not want to see their Habsburg neighbours and rivals inordinately strengthened” (123). Official, publicized justifications coincided with those made in private correspondence. Chapter 4 takes another unique approach, examining post-1648 France and Sweden as guarantors of the Westphalian peace: “For the first time, intervention in the internal affairs of another state became codified in positive treaty law” (132).

Part Two (chapters 5-7) uses case studies of imperial interventions to show how they operated for the smallest territories, small principalities (Nassau-Siegen), and larger territories (Mecklenburg-Schwerin). The Emperor had free rein in the smallest territories, including imperial cities, where violent rule and poor financial management threatened the failure of imperial territories. The chapter on Nassau-Siegen (chapter 6) shows how intervening in even relatively small territories could create difficulties. While essentially all parties agreed that intervention was necessary to stop a violent and tyrannical ruler, jurisdictional jealousies and competing goals among the interested parties complicated the process. Finally, the chapter on Mecklenburg-Schwerin (chapter 7) displays how difficult intervention could be when the subject was a larger, strategically located territory. The Habsburgs negotiated continuously with the Elector of Hanover, the king of Brandenburg-Prussia, and others in order to intervene against a disastrous duke.

A thread running throughout Intervention and State Sovereignty is the book's participation in the ongoing deconstruction of the Westphalian myth. Scholars traditionally argued that the peace originated modern conceptions of state sovereignty and noninterference, yet, as Milton notes, “Westphalia had little to do with sovereignty” (6) and imperial estates already conceived of noninterference as a right. Far from making intervention more difficult, the Peace of Westphalia opened new opportunities for it through its protection of subjects’ confessional rights and the creation of guarantor powers.

There are some issues here. The case studies are generally written in chronological fashion, and with each having so many moving parts in the already-complicated Holy Roman Empire, main points sometimes get lost in the details. This difficulty may be unavoidable given the book's nuanced arguments. There are also quite a few prose errors, mostly missing words and minor misspellings. That said, Intervention and State Sovereignty is a well-researched book offering an important addition to scholarship on the Holy Roman Empire's institutions, sovereignty, political history, and diplomatic history.