In The Iron Princess, Tryntje Helfferich sheds light on a critical figure of the Thirty Years War who has received little attention in Anglophone historiography. While princes such as Frederick V of the Palatinate, Habsburg Emperor Ferdinand II, and Swedish King Gustavus Adolphus are well known to both scholarly and popular audiences, Amalia Elisabeth, widow of Wilhelm V, Landgrave of Hesse-Cassel, has languished in the shadows. This remarkable woman was, nevertheless, arguably the most successful of the German princes who opposed the Austrian Habsburgs. The Calvinist landgravine was certainly the most intransigent, as Helfferich's apt title implies. Indeed, Amalia Elisabeth had an ideal pedigree to take on the Habsburgs and their allies. Her mother was the daughter of William the Silent, and for a short period she herself was engaged to Albrecht Jan Smiřický, who was more famously known as one of the Prague defenestrators and a candidate to replace Ferdinand Habsburg as king of Bohemia. In the end, Amalia Elisabeth married Wilhelm, only to become a widow in exile in 1637. Faced with the enormous challenges of continuing a difficult and expensive war, she doggedly fought to preserve her children's patrimony, to defend the Calvinist cause, and to push back the Habsburg threat to princely independence.

In clear and lucid prose, Helfferich tells the story of how, with a firm hand, Amalia Elisabeth charted Hesse-Cassel's course through these dangerous waters. The landgravine had two important advantages as she continued the war that her husband had initiated. Though displaced from Hesse-Cassel, she controlled an army that occupied a strategic band of imperial territory, including lands that belonged to the electors of Mainz and Cologne. She was also free of the direct financial control of the Hessian estates and could therefore stubbornly resist calls for capitulation or concessions that might compromise her key objectives. Amalia Elisabeth was a master of strategic delay, keeping her enemies at bay and her allies on edge. Helfferich carefully follows her through the last third of the war, when the landgravine had to fend off both an aggressive imperial party and, closer to home, her husband's cousin and rival, Georg of Hesse-Darmstadt, who was eager to claim Hesse-Cassel for himself. Dealing with her allies France and Sweden was equally tricky, as their interests did not precisely parallel her own.

Amalia Elisabeth kept a steady head through the ups and down of the war years and persevered through protracted peace negotiations. The final settlement secured a number of causes for which she had fought, among them a broad reassertion of princely rights within the Empire against the more absolutist claims of the Habsburgs, and the recognition of Calvinism as a protected faith. Moreover, all imperial judgments against the landgravine herself were revoked. She received substantial financial compensation and concluded a notable territorial accord with Landgrave Georg.

This story, compellingly told and nicely paced, is far more than a simple biography. Helfferich, who has also compiled a useful sourcebook on the protracted conflict (The Thirty Years War: A Documentary History, 2009), shrewdly uses Amalia Elisabeth to address at least two related concerns. The first is the war itself, a conflict that was, of course, incredibly complex. Despite a number of recent studies, including Peter H. Wilson's massive overview (The Thirty Years War: Europe's Tragedy, 2009), English-language historiography remains thin, especially for the latter part of the war. The Iron Princess helps guide the reader through the labyrinth of these late years, and Helfferich successfully attacks the old stereotype that the war essentially ended for the German lands after the 1635 Peace of Prague. More generally, The Iron Princess is an accessible introduction to the complicated workings of the Holy Roman Empire in the early modern period. By exploring the tensions between prince and emperor, and their conflicting vision of that supranational state, Helfferich offers key observations on the nature of the Empire and the dynamism of its politics.

There are other important themes as well. Most obvious is the issue of gender and the accomplishments of a woman who, though overshadowed by Queen Christina of Sweden, was in many respects more successful. Helfferich also explores the place and role of Calvinism in the first half of the seventeenth century. At the same time, however, neither the politics of gender nor the inner workings of political Calvinism is investigated in great depth. It certainly would have been useful to place Amalia Elisabeth in the broader constellation of international Calvinism reminiscent of the work of Menna Prestwich (International Calvinism 1541–1715[1987]), or the more recent scholarship of Philip Benedict (Christ Churches Purely Reformed: A Social History of Calvinism[2004]). This is not intended as criticism, however, for authors obviously make decisions about where to place their emphasis. Though Helfferich explores a variety of topics, The Iron Princess is very much a well-written volume of traditional political, diplomatic, and military history. Working through archives in five different countries, Helfferich has painstakingly recreated Amalia Elisabeth's complex story of survival and ultimate vindication. For such an important study, it is a shame that Harvard University Press did not include a bibliography. An overview of Helfferich's extensive archival work and a list of published sources, both primary and secondary, would have constituted a useful guide for future research. A genealogical table, sorting out the tangled lines of Hesse's princely families, would also have been helpful. There are, however, a series of maps that give the reader an important visual overview of both the Empire and the divided lands of Hesse. In sum, The Iron Princess is a model piece of scholarship that highlights both the challenges and promise of investigating Central Europe during the Thirty Years War.