Maritime history is difficult to locate with any spatial precision. Where in the watery expanses of the Atlantic did Columbus’s crews threatened to mutiny? Where off the Azores in 1592 did an English naval squadron seize the fabulously rich Portuguese carrack, Madre de Deus, with its astounding cargo of spices and jewels from the East? Where in the middle passage from Africa to Jamaica did the crew of the Zong send 133 enslaved Africans to their deaths in 1781? Where are the burial sites of the millions of immigrants who died at sea? With oceanic space, there is no way to mark a grave, erect a plinth, investigate the scene of the crime, run a boundary, or post a commemorative plaque.

Since the early 2000s, scholars and environmentalists have used the term Anthropocene to describe a new geological epoch characterized by a discernable human impact on global environmental systems. Within the discipline of geology, the term refers specifically to the ways in which human influence over geophysical systems may leave their mark on the geological record. Almost immediately upon its introduction, however, social scientists, humanists, and activists began to apply the word more broadly to modern humans’ changing relationship to the global environment. Human impact on the natural environment, the term suggests, has reached a sufficient scale to alter long-term geophysical processes on the time scales of human history. Indeed, the industrial era has fundamentally changed the relationships between humans and their material surroundings in ways that we, collectively, are only beginning to understand.

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

As World War II came to a close, the Soviet Union and the United States competed to structure a new order for a world destroyed. Within just a few short years, alliance turned to rivalry, then to enmity. American political leaders came to see the Soviet Union – with its crusading propaganda machine, its vast army, its menacing international behavior, and its repressive centralized state – as an existential threat. Historians continue to debate whether the policy response, “containment,” derived from a realistic or exaggerated assessment of Soviet actions and intentions. There can be little doubt, however, that Americans understood themselves to be reacting defensively to a growing danger. Still, it would be a mistake to conclude that international developments alone drove US policy, for the image of the communist threat was also colored by the domestic context that framed American perceptions of the world.

This volume’s principal goal has been to de-provincialize the study of US foreign relations by exploring how people, capital, things, and knowledge have moved across borders since 1945. This chapter shows how US military members, materiel, and ideas all crossed borders in the first two decades of the twenty-first century, because of – or justified by – national security. The terrorist attack of September 11, 2001 produced a broad militarization of US foreign policy through three different presidential administrations – a pattern that was both a continuation and a departure from previous trends in American foreign relations history. Partly as a result of US actions, what followed were seven wars in Afghanistan, Pakistan, Iraq, Syria, Yemen, Somalia, and Libya – none of which have been fully successful.

Science is an enterprise that deplores boundaries. There is not much point, for instance, in trying to do nuclear physics without chemistry, and it would be foolish for scientists to pretend that they can work in isolation according to state borders, as if (say) mathematicians of different nationalities are somehow engaged in studying separate, national strains of their subject. Ecosystems are not confined by human-imposed lines of demarcation; seas lap on many shores. The Hungarian Jew Michael Polanyi, a polymath educated in Germany and with a long teaching career in Britain, wrote at mid-twentieth century of “a republic of science,” in which scientists worked together across and despite nation-states, trying to solve common problems. “The Republic of Science,” he wrote, “is a Society of Explorers.” Science presented a jigsaw puzzle, the pieces of which could be most efficiently assembled through group effort, each scientist working from his perspective or with her expertise. “The authority of scientific opinion remains essentially mutual,” Polanyi wrote. “Any authority which would undertake to direct the work of the scientist centrally would bring the progress of science virtually to a standstill.”1 The republic required sharing knowledge. It spurned hierarchy. It did not supplant but transcended the state.

In April 1899, prominent Independent Republican Carl Schurz gave a lecture entitled “Militarism and Democracy” to the American Academy of Political and Social Sciences. Treating the “matter of militarism in the United States in intimate connection with our warlike enterprises,” Schurz argued that the nation was at a crossroads: it could enter the Europe-centered world of militarism, war, and empire or continue to keep its distance from that world. In framing this choice, Schurz set the specter of an imperial and militaristic present and future against a virtuous republican past shaped by a miniscule military establishment, purportedly peaceful expansion, and overall distance from European-style bellicism and empire.

In the history of US foreign policy, World War I and its aftermath represents the hinge between the period when the nation played only a marginal role in great-power politics and that in which it has done more than any other to shape them. There were two aspects to this transition. One was the projection of substantial power beyond the Western Hemisphere for the first time. In 1917, the United States embarked on a joint war effort with the Allied powers against Imperial Germany and thereby determined the outcome of World War I. The second aspect was the development of an agenda for the shaping of overseas developments and of the nature of international order generally. This began in 1916 when President Woodrow Wilson made an undertaking to participate on a long-term basis in a postwar collective security organization that was envisaged as the capstone of a liberal international order as well as the guarantor of world peace. Commitment to establishing and maintaining such an international order came in later years to be known as “Wilsonianism,” a term originally coined by its critics but later adopted by its adherents also. In the short term, neither of these developments was sustained. After victory, US forces were withdrawn from Europe and the huge, largely conscripted, army was drastically reduced (if not to prewar size). Then the Treaty of Versailles failed to win Senate approval, leaving the United States outside the League of Nations that Wilson had done so much to establish. Since the 1940s, however, with the renewed projection of American power, now on a longer-term basis, Wilson’s vision has enjoyed renewed influence, though it has never been without its critics.

The economic catastrophes afflicting the United States and the world in the first half of the twentieth century can be divided into several phases but historians generally see connections between them and their shared origins in the cataclysm of World War I. The crisis of 1914–18 brought an end to the intense and deepening late nineteenth-century networks of global trade and, following the peace treaties, economic and other kinds of nationalism flourished. In particular, the United States adopted policies restricting immigration and trade that were especially ill-suited to its new, postwar position as the world’s great lender, manufacturer, and consumer.

“The time has come to think about sex,” the anthropologist Gayle Rubin asserted at the start of a now famous 1984 essay. Sex, held Rubin, might appear to cerebral academics, among others, as “a frivolous diversion from the more critical problems of poverty, war, disease, racism, famine, or nuclear annihilation.” But, she countered, it was precisely in such self-evidently serious circumstances “that people are likely to become dangerously crazy about sexuality.”1

Two overarching contexts shaped American geographies during the era of European overseas expansion: the oceanic networks that connected the Americas to the rest of the world and the continental dynamics by which Indigenous and colonial polities contended for land and power in the Americas.

Human mobility has its own periodization and its own geography. Neither tracks neatly to the more familiar periodization of American domestic politics or the geography of US international relations. While presidential elections, regional conflicts, and international treaties may influence mobility, their precise impact is not always predictable. Migration, developing in tandem with commerce, war, diplomacy, and the movement of capital investment, is always part of globalization, defined here as result of waxing and waning networks of diverse transnational circuits.

Viewing US immigration as one element in a history of globalization pushes attention away from a nineteenth century that began in 1800 and ended in 1899 toward a “long nineteenth century” that began and ended somewhat later. The United States first reported data on migrants in 1820; after 1840 immigration regularly surpassed one million entries per decade. (That means that in the 1840s each year of immigration added 0.6 percent to the American population.)