Despite the publication of many recent works on eighteenth-century Prussia, the earlier history of electoral Brandenburg's rise to power in the seventeenth century still remains comparatively underresearched. This study focuses on an especially neglected topic: the diplomatic relationship between Brandenburg and its northern neighbor, Sweden. Drawing on a large number of unpublished sources from both Swedish and German archives, the book tells the story of two evolving political powers that were connected and sometimes divided by confessional interests, political alliances, and personal networks—all of which contributed to the creation of a complex love-hate relationship over the course of the seventeenth century.

Following recent trends toward a cultural history of early modern diplomacy, the study focuses primarily on diplomatic actors and their practices instead of on wars, political treaties, and the broader structures of power and state-building. It employs a wide understanding of the term diplomat, which includes not only the official diplomatic representatives of a territory (such as envoys and residents) but also semiofficial participants in the diplomatic sphere (e.g., members of the clergy and scholars). According to the author, their specific Irenicist Protestant and cosmopolitan outlook influenced their diplomatic actions and shaped Brandenburg-Swedish relations: the supra-territorial and pan-confessional Protestant cosmopolitanism of Lutheran and Reformed diplomatic actors “could facilitate the relatively free movement of people, ideas and practices,” but could also be “exclusionary and intolerant towards those falling outside of its broadened geopolitical and confessional purview” (11).

Squaring the circle between a culturalist perspective on actors and practices, on the one hand, and a more traditional diplomatic history, on the other, the book covers a period of about one hundred years using a mixture of systematic approaches and case studies. The first chapter deals with the political rise of Brandenburg and Sweden and the establishment of a closer connection between the two states in the decades after 1575. Keeping in mind the impact of neighbors such as Poland, as well as the politics of the Holy Roman Empire, the chapter illustrates the cultural and confessional convergences among leading politicians, which provided an important basis for diplomatic interaction even after the conversion of the Brandenburg Hohenzollern ruling house to Calvinism and Sweden's entry into the Thirty Years’ War (the subjects of the second chapter). A genuinely Protestant Irenicist rhetoric bridged the gaps between Lutheranism and Calvinism, connected both to a broader Protestant cause, and proved particularly fruitful in times of political crisis, leaving space for manifold unofficial diplomatic contacts. This shared outlook was supported by diplomatic actors from Brandenburg and Sweden who enjoyed a similar Protestant educational background, as well as family ties and friendships.

The third chapter consists of microstudies of two marriage alliances between Sweden and Brandenburg in the first half of the seventeenth century: the one between King Gustav II Adolf and Maria Eleonora, the daughter of Elector Johann Sigismund, and the failed marriage project between Queen Christina and Elector Friedrich Wilhelm I (after his accession to the throne in 1640, a pro-Swedish faction seems to have regained ground at the Brandenburg court). This chapter illustrates, in an especially convincing manner, the culturalist approach of the study, which sees early modern diplomacy not as a deficient precursor of the institutionalized and professional diplomacy of later nation-states, but rather as a practice based on individuals, families, and personal networks of specific generations with converging worldviews. The fourth chapter addresses the changing geopolitical situation of Europe from the 1650s, which culminated in the rise of France and a growing divergence between Sweden and Brandenburg over Prussia. The final chapter argues that a growing professionalization of politics, together with generational shifts toward the end of the century, led to the breakup of the older diplomatic relationship between Brandenburg and Sweden, which had been based on Protestant cosmopolitanism. It is ironic that these changes occurred at a time when European Protestants feared more than anything else being marginalized by surrounding Catholic powers. It nevertheless remains an open question why international Protestantism not only survived these changes of the late seventeenth century but also remained an important ideological and rhetorical factor in European politics, even in the first half of the eighteenth-century—with Brandenburg-Prussia still at its center and Sweden slowly moved to the margins.

Unlike other recent studies of early modern diplomacy, this intriguing book takes stock of the intertwined relationship between the two powers—without privileging a Swedish or Brandenburgian perspective. Its “northern” focus fills a gap in the history of seventeenth-century state-building and cultural transfers, and uncovers a story largely hidden by older discussions about a Prusso-German Sonderweg. The notion of an overarching Protestant cosmopolitanism based on Irenicist worldviews might have concealed some fundamental Brandenburg-Swedish differences in Irenicist theology (e.g., issues of predestination, quarrels over the Formula of Concord, differences in ecclesiology)—something the study does not address. In addition, questions of monarchical representation and ideology (such as the Swedish idea of a “Gothicism,” or the political necessity of Reformed Brandenburg rulers to stress their particular “tolerance”), together with the complicated domestic politics of both states, might have shaped the relationship between Brandenburg and Sweden more than this study takes into account. The book is nevertheless a fundamental contribution to the history of diplomacy in the seventeenth century. It is well written, brings together a bulk of Swedish and Brandenburgian archival documents, and connects two national historiographies that have hitherto not been linked as much as they should be.