Nogales is the shared name of two towns in North America that have grown up in the Sonoran desert; they are separated and united by the international border between the United States and Mexico. Until the early twentieth century, people who frequented Brickwood's Saloon in Nogales, Arizona would literally be standing on the border if they moved close to its south wall (it had to be knocked out to make room for a border marker put up in 1894). A lot of alcohol flowed north over the borderline at Nogales during prohibition.Footnote ¹ Much illicit traffic in people and goods continues to flow through Ambos Nogales (as the two towns are collectively known). Yet there is also a tremendous amount of licit traffic that goes through America's Port of Entry at Nogales—$2.5 billion worth of fresh produce—tomatoes, cucumbers, melons, peppers, eggplant, mangoes and more, much of it organically grown. These goods came over the border in Mexican trucks in 2011, supplying a healthy dose of fruits and vegetables to American consumers in the winter months. Meanwhile, Mexican workers from places like Oaxaca make a perilous passage through the desert to enter the United States, heading to fields and groves in California or Georgia or Washington or countless other places to harvest fruits and vegetables for Americans or perform other work vital to the so-called American way of life. Over 6,000 migrants, lost or left behind by coyotes, have perished of dehydration in the crossing in the last decade trying to make it to America's fields and other opportunities for work, as other less environmentally punishing border zones in California and Texas have been hardened with walls and intensive surveillance regimes.Footnote ²

As much as US national politics has driven the fantasy of sealing this border hermetically, the larger history of Ambos Nogales and the entire borderlands between the United States and Mexico is of two regions interwoven culturally, economically, and environmentally through the flows of people and organisms across the border—a two-way traffic in labor and nature.Footnote ³ Placing a border across a sleepy desert drew people, business, and goods to an area that otherwise would have been neglected, placing it on the map of human endeavor. An elaborate apparatus has been built up over the last century to regulate the traffic through this area, at once tasked with interdicting the transborder traffic and keeping it moving. Like other lines between nations, the US-Mexico border has functioned as a “permeable filter.”Footnote ⁴ At Nogales, customs agents regularly discover marijuana tucked in with melons or cucumbers.

Sometimes, the illicit trafficking of nature shows up in more surprising ways. According to a press release issued by the US Department of Homeland Security's US Customs and Border Protection, on September 24, 2012, agents stopped an American crossing north over the border. Inspecting his car, they discovered over a dozen plants. They also found insects in his pockets—and when they opened up a suitcase in the car, they discovered dozens more live and dead insect specimens. Though insects and plants regularly hitch rides on human travelers, sometimes with world-changing consequences, rarely do we find such a literal example of what environmental historian Alfred Crosby called portmanteau biota. With his pockets turned out and his suitcase teaming with insects exposed for the inspectors, the man explained that he was a PhD student at the University of Arizona. Like plant explorers of an earlier period, this man was apparently mining the biological wealth of Mexico, though his motivations appear to be academic rather than economic.Footnote ⁵ His comical crossing, together with the other deadly serious traffic in nature and labor in the borderlands, can serve another scholarly purpose: to highlight the contributions that environmental history has already made to transnational history, as well as the future opportunities the field has to create and to nurture new understandings of the flow of labor and nature across borders.

The calls to open up US history—calls that once seemed like they were made in a vacated forest wilderness—have now reached a thousand pairs of ears. We are seeing transnational studies pop up here, there, and everywhere, as US historiography aggressively attempts to redress decades of work that had entrenched the notion that the United States was a separate, perhaps exceptional, and almost fully sui generis kind of country. No doubt, the isolation of our previous historiography has been greatly exaggerated in order to generate momentum for this “transnational turn”; following on the heels of cultural, spatial, and other turns we've just about come full circle, and we've begun to see more of the world as we've turned around. Scholars in all aspects of American Studies are working hard to reintegrate and reintroduce the United States to the planet at large. From their field's founding in the 1970s, environmental historians have attempted to do the same. Because some of the important early works of environmental history looked past the nation to develop studies that were more broadly pitched and traced the exploitation and trafficking of nature across boundaries, the field, if anything, is now undertaking a transnational return. Still, transnational history and environmental history often seem to be worlds apart. While transnational history conjures images of people moving across porous boundaries, creating identities that are subsumed by no individual national, environmental history still evokes pictures of a static natural world separate from human beings but one that is impacted by them in a one-way manner. Though the fields may appear at first to be running on different rails, environmental and transnational history can and should converge in order to map a shared territory.

Much of the historiographic literature on transnationalism is keyed to issues of definition. Just how is the transnational to be distinguished from, say, the international or the simply national?Footnote ⁶ As Ramón Gutiérrez and Elliott Young see it,

But definitions of the transnational abound and appear to be multiplying. That the “transnational”—like earlier boundary terms such as frontier or contact zone, or categories of analysis like class or race—has denotations and connotations of spellbinding variety should not deter us from recognizing its power and importance. My purpose here is not to settle a difference of meanings of the term. My own preference, though, for defining transnational history is as one that investigates phenomena that traverse national boundaries (including people, goods, ideas, and nature) and that reveals how those crossings are in part constitutive of the places on either side of the boundary lines; those places are only nominally “in” nations since they are not confined or fully inscribed in them due to their transnational implication and involvement. All historians know that national borders are artificial, perhaps never more so when they are formed by natural features such as rivers (which tend to move, in any event, confounding geopolitical lines of demarcation).Footnote ⁸ Environmental historians have long-sought to denaturalize borders, pointing out that they pretend to separate ecologies that are indivisible (though to be sure, certain techniques of hardening can make borders function as effective and disruptive ecological barriers, as with the current US-Mexico border walls).

Transnational historians have decried the social effect of the border fictions made real by nation-states and the United States in particular. Henry Yu suggests,

In fact, joining environmental and transnational history is a key to liberating historians from the prison house of nationalism.

In his laudable effort to urge analytical rigor onto environmental historians speaking of transnational phenomena, Jay Taylor points out that “[w]e should … recognize that [the transnational, along with other] boundary terminology emerged to describe activities primarily concerned with humans, not nature.” “What, exactly,” Taylor asks, “would the terms ‘transnational nature’ or ‘transnational environment’ mean given the centrality of consciousness in the constitution of transnational …?”Footnote ¹⁰ For one, they certainly would contribute to changing the consciousness of historians when we think of nations. The conceptual work the term transnational can do in rendering nations reimagined communities is perfectly pitched by Laura Briggs and her coauthors in their article, “Transnationalism as a Category of Analysis”:

Transnational environmental history, then, has the power and promise to denaturalize the nation. At the same time, it helps focus attention on the state's roles in constructing nature—denaturalizing it to boot. It also can help us trace how the mobility of human beings around the planet and their work in extracting and consuming natural resources have been constitutive of nations themselves as well as the larger formation called, alternatively, global capitalism, the modern world system, or simply empire.Footnote ¹²

Geographers, anthropologists, and political theorists focusing on migration are increasingly interested in the Foucouldian “biopolitics” of borders—how walls and fences, real and virtual, are implicated in the construction of citizenship and legal and illicit traffic.Footnote ¹³ Though used in different senses, biopower can refer to the modern state's interest in cataloging and controlling life from the ground up and from all parts of the globe into their national domains. These controls and efforts of legibility are sharpened as people and nature move across nationally claimed territories. According to Jonathan Inda, since at least the beginning of Operation Gatekeeper in the mid-1990s, the line between the United States and Mexico has become a biopolitical border where “the state creates a wedge between the normal and the pathological, distinguishing between those bodies that can be represented in the polity and those that cannot … implicitly [judging] them to be expendable, suggesting that their lives aren't quite worth living.”Footnote ¹⁴ The biopolitics of borders should be a topic of mutual interest to environmental historians, but we should reframe the “bio” in “biopower” to include the life of nonhumans caught up in transborder traffic as well as issues of environmental justice.

To take stock of the convergence of environmental and transnational history—or what I am calling the transnational return of environmental history—we need to consider briefly national and transnational pathways created by environmental history at its point of origin and then review how labor has been integrated into the field over the last generation. Next, we'll consider a sampling of new works exploring US borderlands—with Mexico, the Pacific, and Canada—that shed light on the integral involvement of the environment and labor in creating transnational phenomena. To map the twined traffic in nature and labor across borders is to map transnationalism.

Environmental history's transnational bona fides are both confirmed and undermined in two of its seminal works. The first is Alfred Crosby's utterly groundbreaking The Columbian Exchange, which demonstrated how the conquest of the new world, with its old world-shaking repercussions, must be understood through an environmental lens. In The Columbian Exchange and his subsequent work, Ecological Imperialism: The Biological Expansion of Europe, 900–1900, Crosby argued that environmental factors, such as disease and invasive plant and animal species, aided and abetted European colonization of the Americas and the world. He also showed how the human-motivated transatlantic movements of plants, animals, and microbes amounted to an ecological reconnection of environments long separated, with profound effects.Footnote ¹⁵ Though much of his work concentrated on periods before the rise of modern states and the transitions from imperial borderlands to bordered lands, Crosby's tracking of organisms that bridged lands and paid little respect to anthropogenic boundary lines helped set environmental history on a course that would be transnational rather than nationally circumscribed.Footnote ¹⁶

But the second work, Roderick Nash's seminal Wilderness and the American Mind, sent environmental history down another pathway.Footnote ¹⁷ Though Nash paid attention to the transatlantic roots of American ideas of nature, his work was aimed at showing how a unique idea of wilderness emerged in the crucible of the United States' experience with nature. Moreover, his goal was to trace a heroic story leading to the creation of the National Parks and argue for wilderness as a cherished American value. As critical as his work was of much of United States' history, it nonetheless championed an American exceptionalism, albeit in an environmentalist caste. It paid little attention to social difference; after all, it took off from the assumption that there could be one “American mind.” It established an important and generative pattern for exploring American environmental history within a national framework. Although a critical revisionist literature has emerged to trouble the celebratory narrative of American ideas and policies of wilderness, some of the best new works on wilderness still hew to mostly national boundaries.Footnote ¹⁸

While non-US environmental historians remained connected to the world in decades that followed Crosby's pioneering work,Footnote ¹⁹ US environmental history reworked the history of the nation in new ways to uncover the role and place of nature in its history. Crosby's planet-wide purview on environmental exchange was used to focus in on New England's history (which has often served, inappropriately, as synecdoche for the nation). Others, fired up by a regional or even bioregional approach, went out to write the environmental history of the South or the Pacific Northwest.Footnote ²⁰ Even as they were archiving locally and thinking globally, these environmental historians were also creating a national environmental history, region by region, of what John Opie (following Perry Miller) called “Nature's Nation.”Footnote ²¹ As Richard White observed in his own call, issued in 1999, for environmental history to operate on multiple scales of analysis in order to include the local, national, transnational, and global, most environmental historians until that time had “de-emphasized the global and transnational and treated environmental issues as largely national.”Footnote ²² No one really believed that their environmental microhistories were in any way closed systems, yet mapping the embeddedness of particular landscapes in material movements spanning borders was not the main aim of these studies—though some of the most ambitious of these regional studies did place their work in a comparative context.Footnote ²³

Another development within the field of US environmental history helped pave the way for its transnational return: an effort to consider questions raised by social historians in order to craft environmental histories that told us more about how the exploitation of nature was intertwined with social dividing lines of gender, race, and class.Footnote ²⁴ If, as C. S. Lewis once wrote, “[w]hat we call Man's power over Nature turns out to be a power exercised by some men over other men with Nature as its instrument,”Footnote ²⁵ environmental historians also sought to develop a fine-grained analysis of labor power in relation to the experience and exploitation of the environment. Richard White, noting a labor lacuna marring the vision of both environmentalists and environmental historians, urged the latter to take the possibility of “knowing nature through labor” seriously. Environmental histories that integrated labor history in sophisticated ways began to appear—tracing the environmental inequalities faced by industrial workers in Gary, Indiana, slaves working in rice fields in Georgia, textile workers in Lowell, miners in Colorado or the Yukon, or farmworkers in California.Footnote ²⁶ In some of these studies, labor history and the history of the body were combined in order to expand the dimensions through which environmental history could place humans as embodied organisms within the natural world.Footnote ²⁷

The attention paid to labor by US environmental historians prepared the field to be reintegrated into transnational history, for many of those people toiling in landscapes claimed by the United States hailed from distant places. Environmental historians were looking at the same people as social historians who were also beginning to trace their subject's experiences and identities beyond the nation's confines. But environmental history has not been able to take the credit for the rise of transnational approaches to the degree that scholars working on migration and the borderlands have. Indeed, most of the hundreds of works dealing with transnationalism as of 2007 were focused on human migration.Footnote ²⁸ Shelley Fishkin, in her presidential address for the American Studies Association heralding the emergence of transnational approaches, noted the “increasing attention to the historical roots of multidirectional flows of people, ideas, and goods and the social, political, linguistic, cultural, and economic crossroads generated in the process.”Footnote ²⁹ In his superb 2012 book, Pacific Connections: The Making of U.S. Canadian Borderlands, highlighting the transnational character of both Asian migration in the Pacific and the transnational bordering practices that sought to control and contain it, Kornel Chang covers the creation of “a far-flung trade network [that] established some of the economic, political, and cultural relationships that gave the Pacific its initial configuration as a region.”Footnote ³⁰ While each author points to the operation of networks of various kinds as an object of transnational study, they pointedly leave out environmental relations and crossroads.

To be sure, borders have become most manifest when they have controlled the movement of people. In fact, the whole process of hardening America's southern and northern borders was set in motion through the attempt to exclude Chinese (im)migrants, which was itself a product of white supremacy and the power of organized labor in the American West.Footnote ³¹ Pekka Hämäläinen and Samuel Truett point out that by looking at “migration networks, economic enclaves, indigenous homelands, world systems, and border zones,” borderlands historians “have staked out a range of alternative perspectives from which the history of America seems less national and more transnational—if not also more transcultural and transregional.”Footnote ³² But this triple historiographic transition—with its attention to the crossings and confluences of cultures, regions, and nations—will not reach its potential if it fails to address how the transborder movement of people was intertwined with the extraction and trafficking of nature as commodities or resources.

In his book Fugitive Landscapes (2006), Truett has done precisely this for the “copper borderlands” encompassing Arizona and Sonora. Tracing how the drive to extract copper further united this transborder region economically, socially, and environmentally beginning in the late 1880s, Truett demonstrates that the international border, instead of separating Mexico and the United States, made them “neighbors by nature.” The drive to extract and process copper to supply wire for the electrification of America, led to the “industrial transformation of the natural world” and “remade a formerly isolated region at the ragged edges of states and markets into an industrial crossroads fed by circuits of capital, labor, and transnational collaboration that extended into both nations.”Footnote ³³ If copper mining brought capital and technology to this region, it was ultimately the transborder mobility of different kinds of workers—among them, US mining engineers, Yaqui and Mexican villagers, Chinese servants and truck famers—that supplied the labor power to make the capitalist transformation of the borderlands work. Truett skillfully draws a “vernacular map—revealing a network of human nodes in the shadow of the copper borderlands.” These include the immigrant labor that the Phelps Dodge Corporation, operating in both Sonora and Arizona, demanded, as well as the “fugitive landscape of smuggling rings, border crossings, and border patrols, places where transnational relationships frequently coincided with illegal practices.”Footnote ³⁴ Truett depicts the efforts of the two nations and transnational corporations to fix the border for their own purposes as constantly being challenged by various actors crossing the border for their own reasons, undermining any fantasy that the border and the borderlands could be entirely controlled.

The history of the US-Mexico border over the course of the twentieth century has oscillated between efforts by (or originating in) Mexico and/or the United States to limit or open up the flows of people and nature as commodities.Footnote ³⁵ The regulation of nature as commodities came first; while copper was exempted from tariffs in the 1890s, American lawmakers put a tax on cattle entering the country. At a few places, such as Mexicali/Calexico and Ambos Nogales, ports of entry were established in order to channel “goods, animals, and other commodities” through them to create “a more rational and manageable economic landscape.”Footnote ³⁶ The rest of the line was officially closed off to traffic. Mary Mendoza points out that “bounding the nation's perimeter has been an attempt to control the movement of not only people, but of animals and other organisms.” She shows how efforts to count and control cross-border cattle traffic served as the schoolhouse for American border control. Designed to protect the domestic space of the United States environmentally as well as economically, the Bureau of Animal Husbandry made cattle entering through designated ports of entry to go through oil-baths to clean them of parasites. In the process of implementing this program of tick interdiction, Mexican cattle were essentially “racialized,” and the Bureau of Animal Husbandry erected the first federally funded fence along the border to separate Mexican from American nature. Consequently, the border has “developed as a complex, often contradictory, attempt to separate people and nature in one place—the U.S.—from the people and nature of their southern neighbor, Mexico.”Footnote ³⁷

Beginning in 1915, an eerily similar prophylactic biological filter was applied to human border crossers. In Eugenic Nation, Alexandra Stern covers the creation, implementation, and social and political ramifications of a border quarantine ostensibly designed to prevent an influx of typhus. Mexican border crossers were subjected to disinfection baths and their baggage was fumigated, rituals that transformed “the boundary line into a construct verified and enacted upon the body itself.” The quarantine, a manifestation of the biopolitics of the border, expressed and furthered the stereotype of Mexicans as dirty and diseased, transforming the border crossing into an “unforgettable passage into the strict racial order of the United States.”Footnote ³⁸ Many of the people subjected to these treatments were on their way to work picking crops in pesticide-treated fields, and they experienced them as biochemical baptisms designed not to renew them in the eyes of a higher power but to dehumanize them at the behest of US authority.

In Migrant Imaginaries (2008), Alicia Camacho uses migrant narratives to explore how the US-Mexico borderlands have been made into a zone of violence and an arena for stripping people of both rights and labor power. She quotes migrants who assert their humanity despite economic and political structures of alienation by proclaiming “De la tierra somos, no somos ilegales [We are of the earth, we are not illegal].” Pointing to the hundreds of women killed around the maquiladoras of Ciudad Juárez and the thousands who are missing and who have been lost in the desert crossings since 1990, Camacho claims that “this violence is not aberrant but central to the current regime of capitalist growth and unequal development. The Mexico-U.S. border is a critical locus for the transnational circuits that form the global political economy and its neoliberal governance.” Materially grounded in the working conditions of Mexicans as a “transborder laboring class,” Camacho's work examines those migrants' works of cultural expression as a way of mapping and hopefully protecting the “fragile social ecology of the transborder space.”Footnote ³⁹ Meanwhile, the very technologies of power erected to mark and police the border have concretized in ways that have harmed both human and nonhuman ecologies. As Rachel St. John explains, “environmentalists and scientists have criticized the fence as an act of ‘environmental vandalism’ and have warned that both their construction and their continuing disruption of borderland ecosystems could threaten endangered species, including ocelots and Sonoran pronghorns.”Footnote ⁴⁰ A full ecological history of the US-Mexico borderlands would encompass the interdynamics of the human and the nonhuman in shaping the waters and lands of the transborder region. But it would do more as well, for the trafficking of nature and labor through the borderlands has been critical to larger systems and flows—including the copper extracted from that earth that conducted electricity across North America and the harvesting of crops in California that ended up on tables in New York or Japan.Footnote ⁴¹

In the early part of the twentieth century, orange growers and other agribusinessmen and women would have seen ads for Peruvian guano in their industry magazines, and many of them would have fertilized their crops with this nitrogen-rich bird waste. As Robert Cushman has shown in Guano and the Opening of the Pacific World, the guano consumed in landscapes of agricultural production in the United States bound those work environments to the greater Pacific, taking us into another critical “borderlands” of the United States. Cushman's work, along with other important environmental and transnational histories of the US and the Pacific “borderlands” written by Ian Tyrrell and David Igler, traces the way that US involvement in the Pacific has opened up transformative channels of contact and exchange between the natures of North America and the Pacific.Footnote ⁴² Cushman concentrates on how the scramble to exploit guano in Peruvian and other Pacific Islands led to the creation of repressive transpacific labor regimes, as Peruvians, Chinese, and Pacific Islanders were set to work to mine the natural resources. Focusing on Hawaiians who experienced the “guano workscape” (adapting a concept developed by Thomas Andrews), Gregory Rosenthal's fine-grained study of Hawaiian laborers harvesting guano and interacting with birds—which he convincingly portrays as workers and actors in their own right—deftly examines the way that these workers not only got to “know nature through labor,” but how they got to know a variety of foreign places as well. Rosenthal notes that “Hawaiian male laborers sojourned across the Pacific world looking for work in a variety of industries, and they held this world—this vast ‘sea of islands’—together through their work, through their experiences of foreign environments, and through their sharing of these experiences …”Footnote ⁴³ In a paper entitled “Whale Riders: Migrant Workers and Migratory Animals in the Pacific World,” Rosenthal extends his analysis to reveal the intertwining of “migration and mobility among both human and non-human actors” in both the whaling and guano industries. Hawaiian workers may not have literally ridden “on the backs of whales as ‘whale riders,’ but they did … follow whales while working aboard whaling ships, charting a unique geography of migrant labor that was determined, in part, by the migration of the animals themselves.”Footnote ⁴⁴ Putting the transnational history of labor and the environment together means both acknowledging the work that nonhuman organisms performed and tracing how human labor migrations spanning out from a variety of national territories intersected with the migratory patterns of whales, birds, or other nonhuman actors.

In the process, thousands of Hawaiians left their islands aboard American ships—chasing sea otters at first and then whales, guano, and other natural resources.Footnote ⁴⁵ As Dorothy Fujita-Rony observes, historians should view “the Pacific not only as a place of journey, but also as a site of work and cultural formation.”Footnote ⁴⁶ David Chang has mapped out how this work-motivated diaspora expanded the geographic imagination of Hawaii and Hawaiians to include the west coast of North America, among other places. Many Hawaiian workers ended up marrying into Native communities from British Columbia to California. Genealogically, these spaces added to Hawaii, even as Hawaii itself was confronting colonizing efforts from abroad, including the United States. Hawaii became a borderland, as multitudes of outsiders came to the islands, from US whale men to Chinese workers. Chang traces the interrelationships among three separate communities (in California, Hawaii, and China) that were spun together bodily, genealogically, and geographically in a transnational web created through the pursuit of natural resources across the Pacific. Chang notes:

If Hawaiians were whale riders entering into new waters and new terrains around the Pacific, they also became riders of logs in the second half of the nineteenth century, when lumber mills in British Columbia, Washington, and Oregon developed a forest-devouring transpacific trade in wood. Working alongside Native Americans, Chinese, Japanese, and European immigrants, these Hawaiians in the US and Canada used their labor power to establish new currents across the Pacific, enabling flows of culture and nature. The milling of trees on an industrial scale drove a succession of changes in the landscape, all of which transformed space in the Canadian West and the Pacific Northwest not only by razing forests but also by opening up landscapes—and waterscapes—to material and cultural influences from across the Pacific. The lumber trade laid down pathways through which the peoples, cultures, and environments of the area were linked to San Francisco, Honolulu, Sydney, Kobe, Shanghai, and other places around the Pacific Rim. In any of those places, people lived in homes crafted from wood shipped out from those distant forests of the “Pacific Raincoast,” passing through the hands of a polyglot collection of migrant laborers from around the world. Workers in the lumber industry maintained and built transnational connections to their home landscapes, whether it was through the Hawaiian sugar or poi you could get at company stores or through the ancestral lands in Japan lost by a migrant and later reclaimed by him with money earned salvaging slab (the waste products of the milling operations) at a steam-powered mill on Puget Sound.Footnote ⁴⁸

The US borderlands with and in the Pacific intersect with the US-Canadian borderlands; though long overshadowed by its southern counterpart, the US-Canadian borderlands are now enjoying serious scholarly attention by environmental and other historians.Footnote ⁴⁹ In The Nature of Borders: Salmon, Boundaries, and Bandits on the Salish Sea, Lissa Wadewitz joins the ranks of those historians who have been moving “beyond the boundaries of the nation-state, venturing across waterscapes, and taking the environmental consequences of border-drawing seriously.”Footnote ⁵⁰ Exploring the waters of the US-Canadian border, which was shot across Native borders, territories, and waterscapes, Wadewitz seeks to uncover the intertwined mobility of nature and of labor by following salmon as they traversed and were pursued by humans in the border waters of the Salish Sea (the new unified name for the body of water called the Georgia Straight above the forty-ninth parallel and the Puget Sound below). Wadewitz observes that “… the imposition of the Anglo-American border was a colonial act completely divorced from issues of salmon conservation … [creating] a spatially, politically, and economically bifurcated fishery …”Footnote ⁵¹ While this border was in part a line drawn by the state, it was, of course, hardly absolute; it was regularly traversed and transgressed, as humans exerted agency to cross the line illicitly for various reasons—sometimes as salmon pirates. In doing so, they displayed the kind of agency that Mae Ngai argues transnational history is best suited to recognize. In fact, Ngai believes that the most important aspect of transnational history is that it “foregrounds human agency. At one level, that is because human movement and practices link—indeed, constitute—transnational spaces.”Footnote ⁵²

But as Wadewitz shows, that border-inflected agency cannot be understood apart from the movement of salmon. Human agents pursuing salmon across borders structured human relations to the environment in critical ways, including accelerating overconsumption of salmon, with various human winners and losers. The border—porous and crossed and crossed again to game the system of regulations—was a “line of opportunity.” Chinese workers, facing racism and official exclusion from the United States after 1882, “finessed the limited flexibility that the border offered in pursuit of more jobs, and higher wages …”Footnote ⁵³ Chinese crossed the line in the water from Canada to the United States, searching for higher wages in its canneries and thus were incorporated into this larger, transnational natural resource industry. Many relied on smugglers to ferry them across, placing them in a precarious position. If, while crossing the invisible line across the water, the smuggler became visible to US Customs agents, he might, like the infamous Larry Pig Iron Kelly, toss his human cargo, weighted with iron, overboard. The modern liberal nation-state has built its legitimacy on the promise of securing rights to life, liberty, and property; but human beings moving through liminal spaces routinely have been stripped of such rights and subjected to violence, suddenly becoming “illegal aliens” or “impossible subjects” and dropped into the dark depths of the Salish Sea or left behind to die of thirst in the Sonoran desert.

The people who have made it across these lines in the sand, or water, have often been searching for jobs in industries directly connected to the natural world, such as mining, agriculture, logging, or fisheries. We need another round of environmental histories of these industries in the United States, ones that place them fully within the transnational realms that they helped constitute and shape. Each industry drew labor from around the world, and the US labor market has been imbricated in and enabled by US imperialist policies abroad.Footnote ⁵⁴ Landscapes of production located within US territory have themselves been transnational sites, as they are powered by human labor and natural resources from elsewhere. In turn, their products—metals, wood, fiber, or food—have been shipped to far-flung national or international markets for consumption. Even as these flows and movements are readily traced by looking at the traffic going across internationally demarcated borders, the transnational return of American environmental history should not limit its purview to these spaces and the resources that can obviously be labeled natural. An orange is not just a thing of nature, but an artifact bearing the inscription of human labor and technology; by the same token, pistols, automobiles, or laptops are not pure products of industry, but elaborate reformations of nature. Moreover, ideas about nature and how to control its travel across borders constitutes another topic open to transnational environmental history. Exploring the processes of assembly, manufacture, and consumption inevitably takes us, again and again, not only across borders of nature and culture, but between this place and state and that.

Whether interacting over land or sea, the environment is a critical force shaping and, in many cases, motivating the movement of people across domains. As people meet and mingle, they bring parts of nature with them—either inadvertently (in the case of microbes, weeds, vermin, and other portmanteau biota) or deliberately (in the case of spices, sea otter pelts, gold, pigs, fruit, drugs, chinaware, earth movers, or insects collected as specimens). In doing so, peoples' movements cross and connect environments. Environmental history has the power to trace the creation and flow of goods in and out of places—including the work that people and their technologies and ideas do to turn nature into commodities for exchange, following where those goods go and whose hands they pass through. It can reveal how far-flung lands were knitted together and transformed through environmental exchanges that were also social and cultural exchanges. Inextricably, nature is part and parcel of all borderlands, and all labor, and all transnational history.

These dynamics are apparent on the ground today at the border in Nogales, one of the places along the 1951 mile border between the United States and Mexico that is bisected with a 20-foot wall. With a group of historians, I recently toured this wall and was able to see first-hand its social and ecological ramifications. Dan Millis, the coordinator of the Sierra Club's borderlands campaign, told us that an impermeable portion of the wall had led to severe flooding in Nogales, Sonora, in 2008, causing millions of dollars of damage and drowning two people. Convinced that the wall is an economic boondoggle and an environmental and social disaster, Millis makes a point of climbing any barrier he comes across—sometimes in view of Border Patrol agents. He was once ticketed for littering. This might seem like an ignominious infraction for a Sierra Club leader, until you find out that his “littering” consisted of putting out water for migrants crossing the arid Sonoran desert. While the Sierra Club has a history of trying to prevent immigration, its current borderlands campaign is focused on reweaving the ecology of a region violently torn asunder by barriers that drive people to their deaths in the desert and strand wildlife on one or the other side of the border. This is a reality that struck home when Millis, working at the time with the humanitarian group No More Deaths/No Más Muertas, discovered the body of a fourteen-year-old girl named Josseline Jamileth Hernández Quinteros.Footnote ⁵⁵

The popular five-term sheriff of Santa Cruz County, Tony Estrada, has lived the borderline. Born in Nogales, Sonora, he moved a mile into Nogales, Arizona, when he was one year old and now describes himself as “someone who has fully realized the American dream.” He remains skeptical of the drive to separate “back-to-back neighbors,” noting that “there is no sealing the border completely.” The polar opposite of Maricopa County's anti-immigrant sheriff Joe Arpaio, Estrada seems both bemused and troubled by the desire to absolutely shut down the border, an impulse expressed by many of his fellow Americans. He notes that most of the migrants display many admired qualities—they are hardworking, family-oriented people of faith and contributors to community. “If we could put all that into a bottle,” he says, “Americans would rush out to purchase it.”Footnote ⁵⁶ What Americans are rushing out to purchase is drugs, as both Estrada and local rancher Tony Sedgwick lament. Sedgwick—an Anglo rancher whose family came to the area from Massachusetts in 1911—owns property that runs right up to the Mexican border. When he comes across drug smugglers passing through his property, toting machine guns, he simply says, “vaya con Dios”—go with God. There is now about 650 miles of fence erected along the border, with widely varying styles but divided into two classes—pedestrian walls designed to deter cross-border foot traffic and vehicle barriers only designed to keep cars and trucks from breaching the border. Near where the Santa Cruz River flows into his land from Mexico, Sedgwick takes us to where the border barrier transitions from the pedestrian style to the “Normandy” vehicle style. As he talks, many of us scramble through the barrier into Mexico. He reminisces about the days before the bifurcation—when he used to meet Mexican horse breeders literally on the border. He'd look the horses over and agree to a price. Most of the vaqueros on his ranch lived across the line in Mexico, and he still meets up annually with his cattle ranching counterparts across the border. He is married to a woman from Nogales, Sonora. He sees and experiences the land and people as a unity—truly “neighbors by nature.” But he is now heart-broken that it is now being fissured and turned into a war zone by what he sees as misguided policies imposed from afar. Looking back to the pedestrian fence—which is constructed of 20-foot tall prefabricated steel bollards, with a space wide enough between each one to reach a hand through but prevent a body from passing—I can see the afternoon sunlight through the slits and the shadows that the bollards cast across the land. The border made physical through steel acts as a kind of prism. It splits and rechannels the movement of people and organisms—of bugs, bodies, microbes, goods, and identities—from one side to the other, but it does not stop the traffic. Mapping the causes and consequences of these refractions is central to the work of a transnationally oriented environmental history.