Interventions in other states on behalf of their subject populations is often portrayed as a novel phenomenon in state practice, one which breaches the old principle of sovereignty. But is this practice really so new? Patrick Milton argues that such interventions for the protection of other rulers' subjects occurred frequently as far back as the sixteenth, seventeenth, and eighteenth centuries. It is the first detailed study of interventions in the early modern period and focusses on central Europe, in particular the Holy Roman Empire. It therefore challenges the common view that in the period after the Peace of Westphalia (1648), the legal scope for, and occurrence of, intervention, were reduced. The book sheds new light on the geopolitical and legal interconnections between the old German Reich and Europe, while also providing comparative insights. It investigates the norms inherent in central European interventions and thereby contributes to a better understanding of the political and legal culture of the Empire, while also assessing the relative importance of geopolitical considerations in such undertakings.