On the eve of the twentieth century, the devoted British imperialist Rudyard Kipling made his first visit to the United States. Arriving at Chicago’s Palmer House Hotel, he found the gilt and mirrored bar “crammed with people talking about money, and spitting everywhere.” Others – he called them “barbarians” – “charged in and out of this inferno with letters and telegrams in their hands.” Outside, the streets of this so-called most American city assaulted the young poet’s senses. He discovered no color or beauty, only dirt for air, drab stone flagging underfoot, and overhead a tangle of wires and “absurd advertisements” for overpriced, inferior goods. Having seen first-hand the “grotesque ferocity” of the Midwest’s largest metropolis, he desired “never to see it again.” Chicago, he said, was “inhabited by savages” who seemed to have no higher purpose than personal profit. Americans, he thought, had yet to develop the will to use their political and economic gifts to earn themselves a place among the world’s leading nations.1

In the era of the two world wars, internationally minded American statesmen turned their attention to meeting two objectives. The first was to engage the United States more with the wider world; more particularly to embed the United States within an international system dominated by European states but, with the inclusion of Japan, becoming increasingly pluralistic. The second was to use this new American global consciousness to reform the international system so that it accorded with the standards of civilization and operated along commonly regarded civilized norms. The codification and promotion of international law was one of the key methods of achieving these objectives; not coincidentally, many, if not most, of the key American internationalists of the era were lawyers.

The country now known and constituted as the United States began life as a series of transplanted colonies of Europeans. They forged new societies along the Atlantic coastline of North America, and they maintained constant contact with their home countries across the ocean. Thus it is little surprise that, for several centuries, the American worldview held Europe as its main ideological and cultural reference point. This Eurocentric perspective has been reflected in the historiography, most recently in the influential Atlantic World school of early modern history which sees the Atlantic coastlands of the Americas, Europe and Africa as an integrated, systemic whole.

Yet the continental United States borders another ocean, the Pacific, that has played an equally important role in the nation's political and economic development. The Atlantic World may have been important, but so too were what Matt K. Matsuda has called the “Pacific worlds”. From the earliest days of the republic, statesmen like John Quincy Adams believed that the United States must expand westward or perish. If the Atlantic marked the natural beginnings of the nation, the Pacific coast was its logical, indeed necessary, conclusion. So important was the Pacific terminus that Adams pursued westward expansion even though it exacerbated sectional tensions over slavery between the North and the South.

Few aspects of the human condition are as complicated as religion. Universal and yet impossible to define because no single definition can capture its diversity and complexity, religion is a source of war and peace, conservatism and liberalism, traditionalism and modernism, nationalism and internationalism, localism and globalism. It is the producer and product of culture, and all that culture entails. Worldwide, in virtually every society, it has provided a basis for the human condition from long before the axial age right through to the present. Yet in the twenty-first century, a digital era of big data, medical miracles, and artificial intelligence, we are more conscious than ever of living in what the philosopher Charles Taylor has called “a secular age.”

For diplomatic historians, the problem of religion has been especially acute. Mostly neglected until the last decade or so, religion poses a unique set of methodological and epistemological challenges. This chapter aims to provide a historiographical overview of religion's burgeoning presence in the field; an introduction to the challenges it poses; and a set of guidelines on how it can be used effectively to explain the history of American foreign relations.

Despite its salience to virtually every time period and most topics in American history, until recently the study of religion had been relegated to the margins of historical scholarship. Instead of being considered an integral component of US history writ large, religious history had developed into a specialist subfield, thriving amongst its practitioners but with little appeal and acceptance in the wider historical community. In an influential essay in 2004, Jon Butler observed that modern American historians had a “religion problem”: they rarely utilized religious actors or subjects; when they did, it was suddenly and temporarily, with usually superficial results. Butler compared religion to a jack-in-the-box, popping up briefly and unexpectedly before disappearing with little trace. Since then, however, political, social, and cultural historians have worked hard to integrate religion into their broader narratives, so that it now has a central role in topics as diverse as civil rights, ethnicity, citizenship, deindustrialization and the emergence of a services-based economy, mass internal migration, and the rise of the conservative political movement. No longer is the historiographical presence of religion ephemeral, sudden, or brief.