This article was written with the support of the Pulitzer Center on Crisis Reporting.

The best handyman living among the boat people in Chong Koh was named Taing Hoarith. Most days, Hoarith woke up at 5 a.m. and bought a bowl of noodle soup from a passing sampan, the same genre of wandering bodega from which his wife, Vo Thi Vioh, sold vegetables houseboat to houseboat. When she left for the day, around 6, Hoarith rolled up their floor mat and got to work.

Chong Koh is one of hundreds of floating villages, comprising tens of thousands of families, on the Tonle Sap River and the lake of the same name in Cambodia. Dangers on a floating village multiply in the rainy season. When I first visited, in late July, there was always something for Hoarith to do: repairing storm damage in a wall of thatched palm, clearing the water hyacinths that collected along the upstream porch. Sometimes the house had to be towed closer to the receding shoreline so that storms or the waves of passing ships would not capsize it. Every few months, he got his ancient air compressor working and swam beneath the house, a rubber hose between his teeth, to refill the cement jars that kept the whole thing buoyant. He was mindful of pythons.

The afternoon of my arrival, Hoarith was squatting over an old butane camp stove, scraping at a rusted gas valve. Rust was the common enemy on the water. Someone had thrown the stove away, but he thought he could fix it to sell on his next trip onto the lake. His wooden long-tail, moored against the house, covered in tarpaulin and heavy with cargo, carried him to floating villages as far as 90 miles away. “I know Tonle Sap like my hand,” he said. There was Prek Tor, a remote village where every family, rich or poor, had a wooden cage for raising crocodiles. And Kbal Taol, where fishermen lived in clustered homes on the open water, risking the daily storms, competing to catch hatchlings with nets up to half a mile long. Hoarith visited them all. He was sometimes on the lake for a month at a stretch, selling pots and stoves, sleeping rough under the long-tail’s planked roof.

But he always came back to Chong Koh, his home of several years, where the villagers live on cabin-size houseboats and junks arranged in tidy rows orthogonal to either shore. In the space between houses, some families raise carp and catfish in bamboo cages or keep floating gardens of potted pepper and papaya trees. Other villages are labyrinthine extensions of nearby shore towns, with broad Venetian canals and twisting alleyways, floating temples, churches, schoolrooms and oil-black ice factories. Chong Koh is relatively small, and shrinking — Cambodian authorities would like it to disappear entirely — but it lies about a mile from the heart of Kampong Chhnang, the large provincial capital, and as Hoarith worked, a steady fleet of peddlers took their boats to and from its markets.

While Hoarith picked at the stove with a screwdriver, a neighbor lay in a hammock, watching him work. The neighbor, like Hoarith and everyone else in the village, was ethnically Vietnamese, and he had a Vietnamese name, Vieng Yang Nang. But most of the time he went by Samnang, which means “lucky” in Khmer, the language of Cambodia’s ethnic majority. Both men kept two names on the water — one Khmer, one Vietnamese — and switched between them freely. They felt at home in both worlds, although they weren’t always accepted in the first. In Cambodia, where the concepts of nationality and ethnicity are inextricable, members of the ethnic Vietnamese minority are known as yuon, a ubiquitous slur that is sometimes translated as “savage.”

Playing xiangqi on the porch of a floating house in the village of Chhnok Trou. Andrea Frazzetta/Institute, for The New York Times

I sat on the floor listening to Samnang and Hoarith revisit a conversation from earlier in the day. That morning we had visited the school and the Vietnamese pagoda, stilted buildings near the fish market where Chong Koh once stood. Local officials evicted the village in 2015, forcing residents to move more than a mile downriver, and both buildings were now hard to access. Such evictions are frequent and unpredictable, and sometimes lead to other trouble. After Hoarith asked the authorities to help with costs related to the move, the police arrested him and accused him of inciting villagers to resist eviction. He spent three months in a squalid prison cell before Thi Vioh borrowed enough money for his release, after which the charges against him were dropped. They only wanted to send him a message, he thought. “I hadn’t committed any crime,” he said. “I had a reputation.”

Hoarith and Samnang agreed that you can’t fight evictions on the water. Floating settlements are technically illegal, and the Vietnamese in particular are powerless against such orders. “The poor will become poorer,” Samnang said. Hoarith said nothing, only swept the pile of rust flakes that he amassed into a knothole in the floor. He set a can of butane into the stove’s empty chamber and pressed the pilot button. A bull’s-eye of blue flame appeared. We laughed. A few houses away, a woman sang love songs on a karaoke system powered by a car battery.

The Mekong River’s lower basin is vast, encompassing parts of Myanmar and Thailand, virtually all of Laos and Cambodia and parts of southern Vietnam, where, after a 3,000-mile journey across five national borders, the mother of rivers divaricates into a complex delta network and drains into the South China Sea. Tonle Sap Lake sits roughly in the middle of this lush expanse. On a map, it appears as a crooked blue finger extending from the Mekong near Phnom Penh. But it is more often described as Cambodia’s heart, both for its rhythmic flood pulse and the sustaining role it plays in the country’s economy and food supply.

Tonle Sap’s unique hydrology makes it one of the most fertile ecosystems on the planet. For half the year, the Tonle Sap River flows southeast from the lake to Phnom Penh. But during the rainy season, the swollen Mekong forces the Tonle Sap to flow in reverse, and the lake engorges to as much as six times its dry-season expanse, two miracles of plenty which over the millenniums have drawn fishermen and rice farmers alike to its doubly silted, nutrient-rich shores. Eels, frogs, shrimp and fish proliferate with tropical abandon, particularly in the fecund bottleneck where, viewed from above, the river appears to fray into dozens of delicate blue fibers before braiding itself back into open water.

The border between Vietnam and Cambodia, which divides the Mekong Delta, has occasioned more battles than nearly any other in Asia. The people living on either side have been in contact for at least a thousand years, an uninterrupted exchange of goods and labor that for the last four centuries has been marked by bold Vietnamese expansion. In the 1630s, a Cambodian king married a Vietnamese princess and allowed the Vietnamese to set up customs ports along the Mekong. The settlers eventually annexed the region, cutting off Cambodia’s access to the South China Sea and stranding many Khmer people inside Vietnam, where they developed a distinct ethnic identity as Khmer Krom. This occupation of “Lower Cambodia” has never been forgiven.

Taing Hoarith, a handyman and stove repairman in Chong Koh. Andrea Frazzetta/Institute, for The New York Times

Cambodia’s borders were formalized when the country became a colonial protectorate in 1863. The French imported Vietnamese workers for its rubber plantations and drew on Saigon’s educated elite as administrative clerks. The number of Vietnamese in the country increased 30-fold, to more than 150,000, or 6 percent of the population. By the time Cambodia declared independence in 1953, the country was polyethnic and multinational, with enclaves of hill tribes and other distinct indigenous minorities, populations of Chinese, Lao and Vietnamese speakers and boundaries that bore only a loose resemblance to those of its precolonial realm. It fell to Cambodia’s first modern leader, Norodom Sihanouk, to unify this haphazardly circumscribed populace.

Sihanouk’s grand idea was to redefine the historic term “Khmer” to include a wide range of ethnic identities. The policy rendered most of the nation’s indigenous groups, including even its Muslim Cham minority, inheritors of the ancient imperial lineage. But the ethnic Vietnamese had no place in this new national typology. Although small numbers of them had lived in the country for centuries, they were Cambodia’s hereditary enemy. After a 1970 coup, they became the targets of pogroms and massacres, adding to the chaos of that decade, which began with civil war and a brutal United States bombing campaign and ended with occupation by the Vietnamese Army. In the interim, during the five-year reign of the Khmer Rouge, millions died from execution, starvation and disease.

Since 1979, the ruling Cambodian People’s Party, or C.P.P. (with the former Khmer Rouge commander Hun Sen at its head), has kept the ethnic Vietnamese in a state of limbo, informally granting and rescinding rights depending on local political climates. The C.P.P.’s opposition, the Cambodia National Rescue Party, or C.N.R.P., is more consistently xenophobic, threatening to expel the Vietnamese invaders and reclaim “Lower Cambodia.” The party’s former leader, Sam Rainsy, once proposed to “send the yuon immigrants back,” and before the 2013 national elections he claimed that “if we don’t rescue our nation, four or five years more is too late — Cambodia will be full of Vietnamese; we will become slaves of Vietnam.”

Human Rights Watch has described the traditional Cambodian hatred of the ethnic Vietnamese as “almost pathological.” It is strongest in the cities, particularly in Phnom Penh, where it can be hard to differentiate between observable corruption and baseless conspiracy. There are legitimate grievances about illegal logging and fishing by Vietnamese companies, but some people also insist that the Vietnamese are to blame for the spread of AIDS or that Pol Pot was a Vietnamese spy sent to annihilate the Khmer race. The hatred is vernacular. Cambodians undergoing gastrointestinal distress say their stomachs are “made in Vietnam,” and the short, prickly tree whose nettles deliver a daylong rash, and which is known to invade a region and quickly overrun it, is the ban la yuon: the Vietnamese barb.

It’s tempting to view the floating villages, where the highest concentrations of ethnic Vietnamese live, as a consequence of politically waterborne lives. In truth, the villages’ history is long and obscure, and no one knows when the first one appeared in Cambodia. The French naturalist Henri Mouhot — who “discovered” Angkor Wat in the 1850s, although it had never been lost to locals — found in Phnom Penh a floating population of 20,000, more than twice as many as lived on land. He described one village of Khmer and Vietnamese merchants just outside the capital:

[T]oward the southern extremity of the city, we passed a floating town, composed of more than 500 boats, most of them of large size. They serve as an entrepôt for some merchants, and residences for others. All their money and the greater part of their merchandize is here kept, that, in case of alarm, they may be ready to take flight at a moment’s warning.

Chhnok Trou. Andrea Frazzetta/Institute, for The New York Times

There is also the collective memory, rarely transcribed, of the floating villagers themselves, who corroborate Mouhot’s intuition that theirs is a lifestyle honed over generations to mitigate against the bad harvests, marauding bandits and unfriendly rulers to which minority Vietnamese remain especially vulnerable. Some told me that they had owned land in the early years of independence and that they had lived on the water only seasonally until the land was taken away. Others said they had always lived on boats. Some identified strongly as Cambodian, while others found the question of national allegiance absurd. “We just live on the water, where it’s easy to catch fish,” a monk in Kampong Chhnang told me. “We lived everywhere.”

Hoarith could count at least four generations of ancestors around Tonle Sap. Born at the mouth of the lake, he was 9 when the Khmer Rouge marched into Phnom Penh. His family was captured and sent to a labor camp in the mountains. After four months at the camp — where, he said, “they tried to kill at least 10 Vietnamese families a day” — the soldiers loaded the prisoners onto ferries to be deported “back” to Vietnam. Hoarith had never been to Vietnam. He didn’t know where Vietnam was. He asked his grandmother, but she didn’t know either. The ferry to the border took five days. Anyone who died was thrown overboard.

Around 150,000 ethnic Vietnamese were expelled from the country in this way, joining the 400,000 or so who had already fled pogroms under the ultranationalist Khmer Republic. The Vietnamese were both regimes’ preferred scapegoat: “ingrate crocodiles” who wanted to swallow Cambodia whole. Those who survived the journey were traded to Vietnam for salt and rice and lived out the disastrous 1970s on farms in the countryside. Those who stayed in Cambodia — 20,000 to 30,000 people — were slaughtered, alongside 90,000 Cham and as many as 100,000 Khmer civilians who were condemned for the crime of having “Khmer bodies and Vietnamese minds.”

When the Vietnamese Army marched into Phnom Penh in 1979 and installed the puppet government that would later become the C.P.P., hundreds of thousands of civilians followed. Many were refugees returning home, but others were immigrants, and their presence revived the same fears of assimilation the Khmer Rouge had stoked. Hoarith and his family resettled in the village where he was born. It was still dangerous. In 1998, during a last gasp of Khmer Rouge resistance, soldiers waded into Chhnok Trou in the middle of the night with R.P.G.s and AK-47s. The village was mixed, so before shooting, they asked: “Are you yuon or Khmer?”

There is a tradition of rural pluralism in Cambodia that belies its recent history of racial violence. Most of the floating villages I saw were peaceful mélanges of Vietnamese, Khmer and Cham fishers, and many of the people I met, including Hoarith, were the product of mixed Khmer and Vietnamese marriages. But everyone seemed to agree that floating villages were traditionally a Vietnamese way of life, enlarged out of economic necessity to include other groups. Today the ethnic Vietnamese live on the water because they are not able to live elsewhere. Neither documented citizen nor, in most cases, immigrant, they are what the government has sometimes described as “nonimmigrant foreigners.” They cannot attend public schools or open bank accounts, get a driver’s license or a factory job or own land or property. Their children are not issued birth certificates, precipitating a generational cycle of de facto statelessness.

A tomb in a Vietnamese cemetery near Chong Koh. Andrea Frazzetta/Institute, for The New York Times

“Thirty years ago, none of this mattered,” Christoph Sperfeldt, a researcher on ethnic Vietnamese citizenship in Cambodia, told me. “No Cambodians had papers. There was no state presence. But the moment the state starts registering people, suddenly it matters.” The expansion of services, including education and health care, and entitlements, including land ownership, has further marginalized those perceived as foreign.

Last year, fearing a narrowing electoral gap, Hun Sen’s government disbanded the C.N.R.P. and arrested Rainsy’s successor on charges of treason. In an effort to defuse the nativism that had fueled the opposition party, the state also began the process of formalizing the status of ethnic Vietnamese as foreigners. Last October, the Ministry of the Interior identified a minimum of 70,000 mostly Vietnamese “foreigners” who possessed “irregular administrative documents.” There may be hundreds of thousands more. Officials began sweeping the country, confiscating IDs and family books and demanding that residents either volunteer to move to Vietnam — where they are similarly considered foreigners — or pay a biannual fee for an immigration card identifying the holder as Vietnamese. “We don’t remove their citizenship; they are Vietnamese,” the head of the country’s immigration department said of the purge. “We just take the Cambodian documents.”

With no formal legal identity and few of the rights enjoyed by their Khmer and Cham neighbors, Vietnamese claim to pay large bribes to the fishery police, the environmental police, the maritime police and other, more ambiguous authority figures, some posing as local journalists. They are subject to evictions, mobs and capricious imprisonment. Hoarith was released in February 2016, but when I met him nearly a year and a half later, the indignity of arrest, for which he blamed his lack of citizenship, continued to occupy his mind. “I should be recognized as Khmer,” he said that afternoon, moving close. His eyes were moss green. “My family has lived in Cambodia for many generations.” He pulled back a curtain in the cramped room, revealing a woman asleep in a hammock. “My mother is 76 years old,” he said. “Even she has no documents.” Nor, now, did he. Upon his arrest, the police had confiscated his birth and marriage certificates and the national ID card he received years ago.

It was agreed I would spend the night on the disused houseboat next door. Some of the weathered boards plunged into space when you stepped on them, and most of the floor was taken up by Hoarith’s supplies: rust-black sheets of tin, a roll of wire mesh, scales for weighing tubs at market. We cleared a spot for bedding, and Samnang and I chased down a boat woman selling old fabrics for a mosquito net.

Dinner was a leathery fish pounded flat and fried in oil on the camp stove Hoarith had fixed that afternoon. Samnang shouted to his wife over the water that he was staying for the meal. Hoarith’s wife and mother ate together on the other houseboat; his mother deftly hopped the gap to retrieve a can of beer from the icebox. She picked up a few grains of rice I spilled from my bowl. “During the Khmer Rouge, this was our entire dinner,” she said. “This was all the food we ate in a day.”

Inside a pagoda in Chhnok Trou. Andrea Frazzetta/Institute, for The New York Times

As night fell, Thi Vioh washed the dishes in a tub of river water, and Hoarith bought bags of bean pudding from the last sampan of the evening, a dessert cruiser strung with colored lights. We listened to the boats roaring up and down the dark channel. Young men in the village liked to modify boats with car engines for night races. Each motor’s throat cleared the air of the insects’ chirping.

It was in one of these brief pockets of stillness that Samnang explained how his brother had died last year when a night racer collided with his fishing boat. His 4-year-old nephew had drowned that year, too, while the parents were both at work. The water has its dangers, including diarrhea (the most common cause of infant death), accidents and drowning. Most children under 5 wear life jackets when their parents can afford them, and improvised devices — the grimmest I saw was an empty motor-oil bottle tied to a length of wire looped around an infant’s neck — when they can’t.

Everyone was in bed by 11 o’clock. From the shoreline came a hollow chorus of empty hulls knocking into mangrove roots. Behind a half wall near my mat, a few alternating floorboards permitted a view of the river between my legs: the facilities. Under the mosquito net, the world was awash in diaphanous pink flowers. Every few seconds, a green light affixed to a roof beam illuminated the room as a warning to nighttime fishermen, whose boats I drowsily mistook for low-flying helicopters.

In the morning, Samnang came back to Hoarith’s to take me to an old cemetery in the flooded countryside. He pulled up to the house in a timber-decked dory with an outboard motor whose steering arm he commanded like a limb. Chong Koh was already bustling. Families were gathered on porches playing xiangqi and eating breakfast, feeding chickens or exercising pigs and dogs along the waterfront. Women drove their children to market while calling out to neighbors what gleanings they sold, whether gasoline, soup or Coca-Cola. Two orphan girls were already hard at work, sitting amid great piles of trei chhlart and squeezing the guts out of each gleaming fish with iron rolling pins.

We drove past the fish market in Kampong Chhnang, where the dock was being hosed down after the morning rush. The village poor, most of them young boys, had moored their wooden taxi boats in the shoals of churned mud and were halfheartedly looking for fares. The water around us was dotted with the crowns of sunken trees and the rooftops of dry-season market stalls. Fishermen who had been out all night were coming home with coolers filled with riel, the silver carp that shares its name with the national currency. The rest were going our way, driving to the systems of nets and bamboo weirs they had installed on the lake according to loose territorial agreements.

At dusk on Tonle Sap Lake, near Kbal Taol. Andrea Frazzetta/Institute, for The New York Times

Samnang was friendly with everyone we passed, although he did not wave unless his hand already happened to be rising to pluck the cigarette from his lips. He had a fisherman’s shrewd economy of movement, and his toes were muscular and permanently splayed, the better to maneuver along the narrow gunwales of a boat in motion. There was an archipelago of pitted skin above his left eyebrow, where he said a drunk Khmer villager had once crushed a glass into his face.

We came to an overgrown graveyard in a raised field. Unlike the Khmer, who cremate their dead, the Vietnamese bury family members in aboveground tombs. Graves thus show a record of Vietnamese residence that no law or loss of document can rescind. Life on the water leaves few ruins, but even families who have immigrated to Vietnam return to Cambodia once a year, usually in March or April, for the annual tomb-sweeping that wards off the wrath of the spirit realm. We drove from cemetery to cemetery, scraping ivy from stones to find the dates beneath, until we arrived at a sunken fen where more gravestones rose from the water like buoys. Samnang pointed to one. “My wife’s grandmother’s,” he said. There had once been a large Vietnamese village here. The stone lay between two converging strands of river. In a few days it would be underwater.

Beyond the cemetery, behind a field of ban la yuon, we found a floating shack in the shadow of a ruined stilted house. A group of children waved from the window. Their father was in the abutting field, stomping through peat that came up to his shins, planting beans. He remembered the Vietnamese who used to live here, on the land as well as the water. There was even a Vietnamese temple — a wat yuon, he said — in a village not far from here. Samnang didn’t know the village, but we decided to go look for it. The farmer warned that we would find no Vietnamese living there. “After the Khmer Rouge,” he said, “they were afraid to come back.”

We followed the farmer’s directions down a narrow stream that meandered for an hour past worm-infested trees from whose branches hung the teardrop nests of tropical birds. Around noon we reached the village, which was called Samraong. There was a dirt road where sickly cows communed with the bovine infinite beneath stilted houses frozen in midcollapse. Children played in the tall grass. Neang Kangrei Mountain loomed overhead, and between the village and the mountain stood an incongruously opulent temple. Its gate was red and gold, framing the mountain’s broad green slope. At the entrance stood a banner pillar, which connects heaven and earth in Buddhist cosmology. The pillar was square, in the Vietnamese style. Two stucco warriors stood on either side of the temple, 12 feet tall, pressing their swords into the earth.

We found an old monk in a hammock outside his hut. He woke up as we approached. Like the bean farmer, he remembered the Vietnamese. They had used this land for centuries, he said. Behind the monk’s house were two brightly painted tombs, whose caretakers traveled from Ho Chi Minh City every March to sweep them.

An aerial view of Kampong Luong. Andrea Frazzetta/Institute, for The New York Times

A group of men had come up the road as we were talking. Now they approached us. The loudest, Uy Poun, boasted to me that he helped to convert the abandoned pagoda 15 years ago, exchanging the Vietnamese gods for Khmer ones. “I built this pagoda,” he said. It had been abandoned for years by then, ever since the Khmer Rouge had moved the whole town to a camp at the foot of the mountain. Some Vietnamese were taken there, and that was the end of their time in the village. “But they weren’t killed,” he added. “They went to Vietnam.”

Another man, Ek Srean, disagreed. Many were killed, he said. “I was an eyewitness. I saw the bones. I saw the bones in the pit.”

“There were bones,” a third said, “but we don’t know if they were Khmer or Vietnamese.”

The men fought over the fate of the Vietnamese for a while. Samnang and I listened and ate some boiled peanuts we bought in the market that morning. Conversations about the Khmer Rouge can have a dreamlike quality in Cambodia, drifting back and forth over the same gruesome territory — the crude methods of murder, the pitiful rations of rice and broth — while trading in rumors, jokes and legends. Facts are overwritten; memories change midsentence. A story is told that contradicts the one preceding it, and both are accepted as passing glimpses of a historical truth too immense to view head-on. After some cajoling, Poun admitted that Vietnamese may have been killed in the camps. “But we didn’t know,” he added. Then he seemed to change his mind. “The Vietnamese never came to the commune where we were.”

We moved on to the question of whether a Vietnamese could ever become Cambodian. As usual, the word “Khmer” was used to denote both ethnicity and nationality. One man in the group put forth a tentative theory: “It depends on their desires, if they want to become Khmer or not. If they give up their Vietnamese nationality, they become Khmer.”

Srean again held a different opinion. “They can hold the documents, but they cannot become Khmer. The Vietnamese are still Vietnamese.” He shook his head and was quiet for a while. “Unless the king signs.”

Samnang, whose Vietnamese name is Vieng Yang Nang. Andrea Frazzetta/Institute, for The New York Times

This generated murmurs of approval. We had forgotten the king’s signature. According to Cambodian law, Prince Sihanouk’s son, Norodom Sihamoni, is the final arbiter of Cambodian citizenship. All applicants for naturalization must meet his personal standards of character. None of us knew whether this had ever actually happened. But the men at least agreed that it might, and that the Vietnamese had lived among Cambodians for generations without any trouble. “Vietnamese and Khmer married each other in this very village,” Poun said. “But not anymore.”

“They were expelled,” another said with finality. “And those who decided to stay were all killed. If they had white skin like a Vietnamese, like that” — he pointed at me, not at Samnang, who was keeping his distance — “they would be killed. I saw it.”

The man seemed to know what he was talking about. Most soldiers and even senior political leaders who served in the Khmer Rouge melted smoothly back into village life after its demise, sometimes rejoining the very communities where they had once worked as executioners. It is rare for anyone to admit such things. The subject was raised in a tactful way. The man who had spoken thought for a moment, then asked to revise his story.

“Well, I didn’t see it exactly,” he said. “I heard about it. But someone who had light skin like you would definitely be killed.”

I spent the next few weeks traveling through floating villages on Tonle Sap Lake and visiting enclaves of ethnic Vietnamese along the Cambodia-Vietnam border. In one border town, I watched a woman use an old land-mine cap as a chopping block. It had been planted in her yard during the Indochina wars and proved itself a dud. In Chhnok Trou, I watched Khmer and Vietnamese neighbors play rousing midnight games of cat te beneath flickering generator light, throwing handfuls of cash into the pot, and everywhere I was the object of extreme village hospitality that knows no ethnic or national distinction: stuffed full of giant river snails and prawn cakes, enticed with can after can of warm Angkor beer.

A child plays with a kite on the roof of a floating house in Chong Koh. Andrea Frazzetta/Institute, for The New York Times

Wherever I went, I asked local politicians and police officers what they thought of the ethnic Vietnamese living among them. The answer depended on where I happened to be. Officials spoke carefully in the cities, and I was politely bounced from the immigration police headquarters in Kampong Chhnang. But I had no trouble at the immigration police station in Kampong Luong, a sprawling conglomerate of floating villages, both Vietnamese and Khmer, on the southern shore of the lake. More than 3,000 ethnic Vietnamese lived in Kampong Luong, alongside as many Khmer, and everyone said they got along swimmingly.

The police station was near the shore, a tin shed floating in a stream of wet garbage. Deputy Inspector Poa Ven was inside the shed, sweating. He was happy to see me. “We are aware of all foreign visitors,” he said, chuckling, when I handed him my business card.

All the foreigners in the village were Vietnamese nationals, he explained. There was no difference between an ethnic Vietnamese and a Vietnamese citizen unless they got a letter signed by the king. “No one here has Khmer nationality.”

I explained my confusion. Most of the people I’d met on floating villages could trace their lineage in Cambodia back many generations. They spoke Khmer fluently and had even been issued IDs by previous administrations. They had no ties to Vietnam.

Yes, he said, even they were Vietnamese. Those were the new orders from the Ministry of the Interior.

The kitchen inside a floating house in Chhnok Trou. Andrea Frazzetta/Institute, for The New York Times

“But why are they Vietnamese?”

“Because they are still immigrants! Because they came to Cambodia in 1980, they are still immigrants.”

“But if they were originally from Cambodia, why are they immigrants?”

“Because during the civil war, they went to Vietnam, and after the war they came back.”

Nationalism is always in search of an enemy. In Cambodia, the search has a neat circular logic: The Vietnamese are enemies because they are foreign. They are foreign because they are enemies. Their existence here betrays a contradiction between the myth of a pure Khmer empire and the country’s lived history of migration and movement along the Mekong. The contradiction has been resolved at times by violence, but it is perpetuated by education. The Khmer Rouge years may be fading from living memory, but children are still taught to resent the loss of “Lower Cambodia,” and every student has heard of the rapes, gas attacks and elaborate acts of torture that Vietnamese soldiers are believed to have committed during the Indochina wars.

“All of that stuff tends to weigh very heavily on Cambodian minds,” Craig Etcheson, a Khmer Rouge scholar at Harvard, told me. “And that is very easy for opportunistic politicians to exploit and inflame.” Etcheson is a founder of the Documentation Center of Cambodia, which has provided much of the evidence for the Extraordinary Chambers in the Courts of Cambodia, or Khmer Rouge Tribunal, convened in 2006 to try a handful of senior Khmer Rouge leaders for crimes against humanity. The second phase of the trials, concerning genocide against minority groups, including the Vietnamese, concluded its final arguments last summer. A ruling is expected this year.

Lyma Nguyen, an international civil-party counsel at the tribunal, told me she had hoped that one consequence of the trial would be a pathway to a stable legal identity for Vietnamese survivors of the Khmer Rouge. But she encountered insurmountable resistance to the idea. Current plans for reparations include only a watered-down education program for raising awareness about nationality laws. “Many mainstream Cambodians, including some lawyers and academics, actually don’t think the Vietnamese victims of the genocide should have a legitimate claim to having suffered genocide, because they’re Vietnamese,” she said. “They think it’s all a big conspiracy by Vietnam to swallow up Cambodia.”

Yao Bu Dung makes her way home through the marsh with her child before a storm reaches Kampong Luong. Andrea Frazzetta/Institute, for The New York Times

Most ethnic Vietnamese in the country continue to feel that they are Cambodians of Vietnamese origin. They refuse to give up hope that someday their Cambodian identity will be accepted in the country they call home. Instead, with somewhere between 400,000 and one million members, according to independent scholars, and virtually no international calls for Cambodia to uphold its own nationality laws, they are arguably one of the largest and least-supported stateless populations in the world.

The ethnic Vietnamese have taken to hiding their documents from the police currently sweeping the country. It’s not the first time this kind of thing has happened to them. At the Vietnamese pagoda in Kampong Chhnang, I met a diminutive 80-year-old seamstress. Only monks and laypeople are meant to live at the pagoda, but she didn’t have any family — her children died before her eyes in a Khmer Rouge labor camp — so the monks took pity on her. I asked whether she ever received any identification papers. She shuffled to her room and came back holding a small packet wrapped in twine. “You’re lucky,” she said as she untied it. “I’ve never let anyone look at these before.” She began spreading an astonishing half-century’s worth of documents across the table. There were cards from Sihanouk’s Cambodia alongside decades’ worth of residency permits from Vietnam, to which she was twice deported. So long as she never showed them to anyone, the documents could never be invalidated or purged. They were her private legal self, a superposition of identities both Vietnamese and Cambodian. She would not give them up. “I’ll keep these documents with me until I die,” she said, “and then I’ll take them to the grave with me.”

The monks on the floating pagoda in Kampong Luong were preparing for the ceremony of the drowned, arranging yellow orchids and four-o’clocks beneath a Buddha framed in flashing lights. A rented pavilion had been floated over and stocked with chairs. Young volunteers scattered riel and mangos on a picnic table, and all morning long pilgrims from Phnom Penh arrived to give money to the poor and receive blessings. It was August, and the air was torpid.

Drowning is a bad death, the temple laymen explained. The souls of the drowned become water ghosts — khmoch teuk in Khmer — causing shipwrecks and pulling swimmers under by their legs. The ceremony coaxes the spirits out of the water so that they may find their way to the next life or proceed to the heavenly plane.

The heat broke with a late-afternoon downpour. The children of supplicants made a game of leaping through the deluge onto an empty fish barge moored to the pagoda, then back again with fearless precision. At dusk, the ceremony began. There was an hour of amplified chanting before the head monk tossed holy water over the crowd and called for the doors of hell to open. As if on cue, the rain slowed, then stopped. The temple emptied into the ferries and sampans outside. At the front of the largest ferry, a monk rang a finger-cymbal to wake the drowned, and the boats slipped onto the open lake. There was no moon. The water was black. Webs of distant lightning soundlessly limned the clouds red. Along the sides of the ferries and the gunwales of each boat, families placed paper rafts and plastic lotus flowers into the water. Each was topped by a burning candle, such that the lake became a field of bobbing orange stars. All was still, save for the families who boated past, singing into the dark. Some of their boats nearly spilled over with children. Some of the paper boats flared up in a flash and sank.

At the ceremony for drowned spirits in Kampong Luong, families place plastic lotus flowers and paper boats in the water. Andrea Frazzetta/Institute, for The New York Times

Beyond the scarp that runs along Chong Koh, a new market complex was under construction, and with it came berms of red soil and cement foundations cratered with silted ponds. Once the market was finished, the villagers in Chong Koh would be evicted again, ahead of floating communities across the province. Officials said the villages were illegal and environmentally toxic. Their residents would be squeezed onto marshy rented plots earmarked for immigrants, far from their boats and fish cages.

Neither Hoarith nor Samnang wanted to move. “I can’t speak out,” Samnang said as we sat drinking coffee in his house. “They claim to give us a choice. But we have no right to buy any other land, so really it’s no choice at all.”

We paddled next door, where Hoarith was too consumed by boat repair to worry about any new evictions. Last week he had chopped three feet off the back of his long-tail. Now his transmission was spread out before him like a gutted squid.

As he worked, Hoarith’s thoughts often went back to his prison cell and to the spot of cold floor next to the toilet where he had slept. “I never discriminate against anyone, Vietnamese or Khmer,” he said. “But I was treated so badly.” If he were granted Khmer citizenship, he said, all would be forgiven. “My parents were born here, and I was born here,” he said. “I have that right.”

There was nothing to say. Samnang found a hatchet to sharpen on the rear step over the water. Hoarith turned back to his work. A washer, slick with oil, shot from his hand. It bounced against the floor and off the side of the house before he caught it over the water, hissing in surprise.

The first people to leave Chong Koh went by night as soon as the new immigration cards were announced in 2014. Since then, at least a thousand have shoved off to other provinces or sold their houses and hitched to Vietnam, part of a growing exodus all over Cambodia. In village after village, the immigration police and council leaders said the same thing: “The Vietnamese are leaving.”

In a pinch, Hoarith thought he might be able to live with his wife’s relatives in Vietnam. Better was the lake. On the open lake it was more dangerous, but he would be close to his ancestors, and the authorities there weren’t as strict. The boat people had figured out how to manage them, as they managed the squalls and the waves. They knew how not to challenge the weather but survive it, was how he put it. That was the trick. You had to plan for the storm before the clouds opened up.