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.