At the end of the nineteenth century, the United States of America oversaw both a formal empire with colonies as well as an informal empire, the latter of which reflected economic and military policy and practices. Terms such as “colony”, “empire”, and “imperialism” remain loaded and contested within the historiography of US imperialism. Ian Tyrrell's well argued and well researched book adds to the scholarship on this period of US history through examining the contributions of moral reformers to the informal US American global expansion as well as to the transnational networks associated with this expansion. Moral reformers in their various and eclectic guises, Tyrrell notes, have been an understudied aspect of the US American imperial expansion. In examining their work in overseas locations from the 1880s to the 1920s, Tyrrell creates an image of a dynamic, transnational movement in which individuals or groups intent on morally transforming the world transformed themselves into highly organized, commercial operations in which the voice of American moral reform was clearly propagated, and which effected change in the United States itself. Throughout the book, protestant missionaries play a prominent role, reflecting a burgeoning interest in the work of missionaries in broader academic circles. Catholic missionaries do not receive much attention, as, during this period, Catholics saw America as a mission site itself and were not so actively engaged in overseas missions. Missionaries are, however, not the only actors within this book, and Tyrrell also turns his attention to non-denominational organizations such as the Red Cross, as well as individuals such as Mary Leitch and John Mott and thereby, as the title indicates, to the broad genesis of the American moral reform movement. Through examining the stances of moral reformers in regard to prostitution, opium, and alcohol use in American colonies, especially in the Philippines, as well as the political clout held by moral reformers in America – manifest for example in their support of prohibition laws in post-world War I America – Tyrrell weaves together a colorful tapestry of people and organizations which helped shape the moral and political landscape of America's domestic and foreign policy.

Tyrrell's self stated goal is to broaden the “context of the drive toward American imperialism – situating it within wider patterns of informal American expansion and the transnational networks implicated in those patterns” (p. 2), and it is indeed his attention to moral reformers and their often innovative ways of spreading their messages which adds layers of complex detail to his argument. He, for example, follows the broader expansion of American cultural exports not only through clearly defined American moral reformers, such as missionaries and members of the temperance movement, but also through tourists, popular culture, and sports groups. Although many of these phenomena formed implicit elements of broader American cultural expansion abroad, Tyrrell's exploration of the cultural and organizational changes that occurred within the American voluntary reform movement focuses more on America itself, rather than changes effected in overseas territories.

His focus is thus placed upon the effects that the moral reform movement, which was directed towards a non-American audience, had upon America itself. This is particularly evident in Chapter 8 where Tyrrell follows the work of such moral reformers as the African-American Ida Wells and her work in Britain amongst moral reformers there to raise oppositional voices against lynching in America. Such examples are used to demonstrate how international moral reform networks were utilized to effect change within America and, in this case, how they were used to focus international attention onto a domestic manifestation of American racism.

Tyrrell crafts a narrative that demonstrates the interplay between the international and national, revealing how the American moral reformers not only effected change in the extra-American world, but also how in doing so they effected change in their own groups back home. For example, he describes in Chapter 4 how the British-founded Young Men's Christian Association (YMCA) was exported to America in the 1840s and became popular with middle-class Protestants. During the religious revivalism period of the 1880s, the impetus was in place for the American YMCA to expand internationally. In doing so the movement brought an American form of cultural imperialism to places such as the Philippines, where YMCA gymnasiums, often themselves supported through funds from rich businessmen, underscored a notion of modernity and practicability whilst shaping a “new colonial man, modeled on Euro-American ideas of a muscular Christianity” (p. 88). American moral reform groups such as the YMCA saw themselves in opposition to similar British groups that were perceived to be staid and conservative, and in contrast to this perceived conservatism the YMCA followed a path of aggressive evangelicalism bound up with notions of Westernization, in response to which they were in turn criticized by European religious organizations for being overtly enthusiastic and centered upon quick results. In other locations, however, the YMCA had to adapt to local situations, which often meant a focus upon social services as opposed to evangelicalism. It is through exploring such issues of changing identity and personal networks that Tyrrell approaches the predominant theme of his book: a demonstration of the complex relationships between the national and the transnational.

The book is divided into four parts entitled: Networks of Empire; Origins of American Empire; the Challenge of American Colonialism; and the Era of World War I and the Wilsonian New World Order. Given the broad range of topics covered and the multitude of different actors within the 250-odd pages of text, it is to be expected that broader historical context is sometimes provided only cursorily. For example, Tyrrell sometimes assumes familiarity with international aspects of American colonial politics that risks bewildering readers less familiar with the book's background.

Tyrrell's discourse is focused not just upon what could be termed the 'bleeding-heart' moral reformers, rather he juxtaposes humanitarian motives with commercial and political ones, demonstrating that the motives for American humanitarianism abroad were complex, producing historical acts able to be variously—and even simultaneously—described as genuine, self-serving, guilt-laden, and imperialistic in nature. Tyrrell, for example, argues in Chapter 7 that the moral reformers' opposition to the opium trade in the Philippines helped “fashion and support wider American economic foreign policy in the region in ways that legitimized the growth of American world power” (p. 157), and that, through taking the high moral ground against Britain and other European powers, American politicians could demonstrate their exceptionalism and thus justify their hegemonic power within broader Asia. Although the book threatens on occasion to gravitate towards broad, sweeping arguments, Tyrrell manages always to return to the facts at hand and to present a broader context which allows the protagonists' actions to be seen in the complex social, political, and moral environments in which they acted.

This book is well crafted, and the multiple threads laid out at the beginning are carefully and subtly woven into a tight and coherent narrative, allowing the reader to enjoy the thrill of recognition as well as the blossoming awareness of the entangled nature of the moral reform movement in American imperialism. This is no easy task and Tyrrell has managed to create a book full of tensions and questions which the reader is drawn into, engages in, and emerges from with a broader understanding of, and critical insight into, this phase of American imperialism.