Addressing the graduating cadets at West Point in May 1942, General George C. Marshall, then the Army chief of staff, reduced the nation’s purpose in the global war it had recently joined to a single emphatic sentence. “We are determined,” he remarked, “that before the sun sets on this terrible struggle, our flag will be recognized throughout the world as a symbol of freedom on the one hand and of overwhelming force on the other.”

At the time Marshall spoke, mere months after the Japanese attack on Pearl Harbor, U.S. forces had sustained a string of painful setbacks and had yet to win a major battle. Eventual victory over Japan and Germany seemed anything but assured. Yet Marshall was already looking beyond the immediate challenges to define what that victory, when ultimately— and, in his view, inevitably—achieved, was going to signify.

This second world war of the twentieth century, Marshall understood, was going to be immense and immensely destructive. But if vast in scope, it would be limited in duration. The sun would set; the war would end. Today no such expectation exists. Marshall’s successors have come to view armed conflict as an open-ended proposition. The alarming turn in U.S.–Iranian relations is another reminder that war has become normal for the United States.

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A man is to carry himself in the presence of all opposition, as if every thing were titular and ephemeral but he.

I rose long before dawn, too thrilled to sleep, and set off to find my tribe. North from Greenville in the dark, past towns with names like Sans Souci and Travelers Rest, over the border into North Carolina, through land so choked by kudzu that the overgrown trees in the dark looked like great creatures petrified in mid-flight. The weirdness of this scene would, by the end of the weekend, show itself to be appropriate: my trip would be all about romanticism, and romanticism is a human collision with place that results, as Baudelaire put it, “neither in choice of subject nor exact truth, but in a way of feeling.” My rental car’s engine whined as it climbed the mountains. Day was just breaking when I nosed down a hill to Orchard Lake Campground, where tents were still being erected in the dimness.

Harold Jamieson, once chief engineer of New York City’s sanitation department, enjoyed retirement. He knew from his small circle of friends that some didn’t, so he considered himself lucky. He had an acre of garden in Queens that he shared with several like-minded horticulturists, he had discovered Netflix, and he was making inroads in the books he’d always meant to read. He still missed his wife—a victim of breast cancer five years previous—but aside from that persistent ache, his life was quite full. Before rising every morning, he reminded himself to enjoy the day. At sixty-eight, he liked to think he had a fair amount of road left, but there was no denying it had begun to narrow.

The best part of those days—assuming it wasn’t raining, snowing, or too cold—was the nine-block walk to Central Park after breakfast. Although he carried a cell phone and used an electronic tablet (had grown dependent on it, in fact), he still preferred the print version of the Times. In the park, he would settle on his favorite bench and spend an hour with it, reading the sections back to front, telling himself he was progressing from the sublime to the ridiculous.

1. In 2014, Deepti Gurdasani, a genetic epidemiologist at the Wellcome Sanger Institute in England, coauthored a paper in Nature on human genetic variation in Africa, from which this image is taken. A recent study had found that DNA from people of European descent made up 96 percent of genetic samples worldwide, reflecting the historical tendency among scientists and doctors to view the male, European body as a global archetype. “There wasn’t very much data available from Africa at all,” Gurdasani told me. To help rectify the imbalance, her research team collected samples from eighteen African ethnolinguistic groups across the continent—such as the Kalenjin of Uganda and the Oromo of Ethiopia—most of whom had not previously been included in genomic research. They analyzed the data using an admixture algorithm, which visualizes the statistical genetic differences among groups by representing them as color clusters. The top chart shows genetic differences among the sampled African populations, in increasing degrees of granularity from top to bottom, and the bottom chart shows how they compare with ethnic groups in the rest of the world. The areas where the colors mix and overlap imply that groups commingled. The Yoruba, for instance, show remarkable homogeneity—their column is almost entirely green and purple—while the Kalenjin seem to have associated with many populations across the continent.

Ten yards was the nearest we could get to the river. Any closer and the smell was too much to bear. The water was a milky gray color, as if mixed with ashes, and the passage of floating trash was ceaseless. Plastic bags and bottles, coffee lids, yogurt cups, flip-flops, and sodden stuffed animals drifted past, coated in yellow scum. Amid the old tires and mattresses dumped on the riverbank, mounds of rank green weeds gave refuge to birds and grasshoppers, which didn’t seem bothered by the fecal stench.

El Río de los Remedios, or the River of Remedies, runs through the city of Ecatepec, a densely populated satellite of Mexico City. Confined mostly to concrete channels, the river serves as the main drainage line for the vast monochrome barrios that surround the capital. That day, I was standing on a stretch of the canal just north of Ecatepec, with a twenty-three-year-old photographer named Reyna Leynez. Reyna was the one who’d told me about the place and what it represents. This ruined river, this open sewer, is said to be one of the largest mass graves in Mexico.