Chico Mendes was hard to kill. No one knew that better than his enemies in Brazil’s Amazon rain forest. They had failed in half a dozen assassination attempts.

But on a December night in 1988, Mr. Mendes’s luck ran out. A shotgun blast ripped into him as he stepped outside his wood-frame house in the western Brazilian state of Acre. It was the end of a man who had won global acclaim for championing the sanctity of the forest and the rights of compatriots who eked out a living by extracting latex from rubber trees.

Retro Report, a series of video documentaries that explore the enduring impact of major news events of the past, harks back to the eco-martyrdom of Francisco Alves Mendes Filho: Chico to everyone. More broadly, this episode looks at the perilous state of tropical forests — notably the Amazon, the biggest of all at 2.1 million square miles — and the threat that persists for indigenous peoples and for the environmental balance of the planet.

Over the last half-century, roughly 20 percent of the Brazilian Amazon has disappeared as a result of deliberately set fires and relentless bulldozing to make room for cattle ranchers and to clear paths for loggers, road builders and other developers. Mindful of the international outcry over looming ecological disaster, Brazil has tried to slow the pace of this deforestation, with a commitment to bring it down to zero by 2030. But steady progress has proved elusive; of late, there has been some backsliding. Environmentalists warn that in another 15 years or so, little of the rain forest may be left to be saved.

The Amazon, nearly two-thirds of which lies in Brazil, is often described as the world’s lungs because of the carbon dioxide it absorbs from the atmosphere and the oxygen it pumps back out. Cutting down trees, not to mention burning them, upsets the normal process. And Brazil is not the lone concern. Indonesia, the journal Nature wrote last year, “is clearing more forest than any other country.” By some estimates, deforestation worldwide accounts for about 15 percent of greenhouse gas emissions.

Mr. Mendes’s death at age 44 became a turning point in Brazil’s environmental consciousness.

Unable to read until he was 18, he started out as just another guy trying to get by. He worked as a rubber tapper, one of many in the Amazon who cut into trees and collect the latex oozing from the incisions. The methods are a circumspect intrusion on nature, allowing for constant renewal.

Deforestation threatened the rubber tappers’ livelihood. With his social conscience aroused, Mr. Mendes helped unionize them. He organized resistance in the form of human chains that blocked developers’ incursions into the forests. Eventually, he came to appreciate that saving the Amazon was a blessing not only for his workers but also for the world.

But while international environmental groups lauded him, Brazilian cattle raisers and others with a financial stake wanted him out of the way. “He was to the ranchers of the Amazon what Cesar Chavez was to the citrus kings of California, what Lech Walesa was to the shipyard managers of Gdansk,” Andrew C. Revkin, who long covered environment issues for The New York Times, wrote in “The Burning Season,” his 1990 book on Mr. Mendes’s life and death.

Murder in the Amazon was hardly unfamiliar. Across a quarter-century before the Mendes shooting, 982 environmental activists were killed in land disputes, the Roman Catholic Church in Brazil said. Most died at the hands of ranchers’ hired guns. The killers brought to justice could be counted on one hand. Mr. Mendes, though, did not become just another statistic. His murderers — father and son ranchers — were found guilty and sentenced in 1990 to 19-year prison terms.

In the aftermath, a movement to save the Amazon gained impetus.

Brazil’s leaders now saw merit in putting the brakes on deforestation. They created preserves in the Amazon basin, with moratoriums on tree-clearing. (One preserve, covering 3,750 square miles, nearly the size of Delaware and Rhode Island combined, was named for Mr. Mendes). Law enforcement officials cracked down on rogue ranchers, loggers and land speculators.

Plans were halted for a dam that would have flooded territory occupied by the Kayapó tribe. International businesses stepped up as well. Several large beef producers agreed to boycott ranchers whose herds grazed on denuded stretches of the Amazon. Giants like Nike and Gucci, perhaps reacting to customer pressure, agreed to have nothing to do with leather from razed sections of the forest.

By the early 2010s, the rate of Amazon deforestation was one-fourth what it had been a decade earlier. It was part of a worldwide trend. In 2015, the Food and Agriculture Organization of the United Nations reported that across the planet, 129 million hectares of forest — nearly 500,000 square miles, an area roughly equivalent to that of South Africa — had disappeared since 1990. Nonetheless, the agency said, the annual rate of loss had slowed appreciably as forest management improved.

Staying the course has proved difficult, in part because Brazil landed on hard times and has re-emphasized economic development. Recurring turmoil, including the impeachment and removal of President Dilma Rousseff in August, raises questions about the political will to pursue environmentally friendly policies.

Forest burn rates spiked in 2013 and again in 2015; preliminary data for the first half of this year suggest that the trend continues. The dam project opposed by the Kayapó sprung back to life, going into operation this year. Gold mining expanded. Officialdom relaxed rules that limited deforestation and reduced the percentage of the Amazon required to be preserved. Cattle ranchers found guilty of unlawfully clearing trees were granted amnesty.

As for murders of environmental activists, they never stopped. One prominent victim among many hundreds was Sister Dorothy Stang, a 73-year-old American nun shot to death in 2005 by ranchers’ gunslingers. Brazil is not alone in this drumbeat of death. Global Witness, a watchdog group based in London, reported 185 such killings around the world in 2015, an average of one every other day. Those were just the killings that could be documented; the true toll, the organization said, was probably much higher.

Latin America is singularly hazardous for activists. Brazil had 50 such killings last year. Colombia had 26, Peru and Nicaragua 12 each, Guatemala 10 and Honduras eight. On the other side of the globe, the Philippines had 33.

These setbacks notwithstanding, many Brazilian ranchers seem to grasp the importance of sustainability. Steve Schwartzman, who is in charge of tropical forest policy for the Environmental Defense Fund, told Retro Report that “a basic insight, which goes back to Chico Mendes, has become commonplace — to make standing forest a real economic asset that benefits the people that are living in it and that creates real growth without environmental disruption.”

Mr. Mendes understood it decades ago. “At first I thought I was fighting to save rubber trees,” he said a year before his death. “Then I thought I was fighting to save the Amazon rain forest. Now I realize I am fighting for humanity.”