Such was the experience of José Rosa, a rancher in the frontier town of Matupá, 20 miles south of Guarantã do Norte, who had discovered that grileiros were cutting trees on his property. It's not that Rosa objected to the idea of clearing land—he himself plans to plant 2,500 acres in the coming year—it's just that someone else was blatantly trying to steal his. Despite federal pledges for more resources to combat timber mafias and land sharks, the only help Rosa could round up was a tiny posse of two IBAMA agents and a local cop. Among them, they carried a single pistol and a pump-action shotgun—not much of an arsenal against heavily armed grileiros. To buy gasoline for their pickup truck, the IBAMA agents had to dig into their own pockets.

Evanoir Tibaldi, 42, the commander of this ad hoc detail, has spent 15 years working for IBAMA on the front lines in northern Mato Grosso. When I asked about the satellite imaging system that is supposed to give field agents the data they need to catch grileiros red-handed, Tibaldi replied, "We don’t even have Internet in our office—it's a joke."

Rosa, in his grimy red sport shirt and battered hat, didn't look the part of wealthy fazendeiro, or plantation owner, with an 18,000-acre spread and 3,500 steers. Getting to his land required a two-hour drive east from town, down a dirt road and across flat plains and rolling hills, where blocks of forest still stood amid brilliant green fields of rice and soybeans. "The land here is perfect for soy," Rosa said.

On his property, we headed uphill through fenced-off pasture and entered the darkness of the forest along a two-rut road made by grileiros. We crossed a stream, so clear and inviting that we stopped for a drink. As I beheld the green cathedral that towered above us, I had the sense that we were day-tripping in a sacred place that should have taken weeks of arduous trekking to reach. An iridescent blue morpho butterfly lilted past, one of a million wonders still harbored by this primal forest. But for how much longer? Recalling the murky stew I'd seen in streams already overrun by farmland farther south, I figured it would be only months—not even a year—before these deep, mysterious shadows were exposed to scorching sunlight and the cool, clean water no longer fit to drink.

Bouncing along washed-out tire tracks overhung by low branches, we suddenly emerged onto a wider, newly graded road. "These aren't poor people doing this," Rosa said. "These are land grabbers. They have a lot of money. If they find me out here alone, they will kill me."

The invaders were brazen enough to have erected and locked a gate across the road. We proceeded on foot. Tibaldi signaled for silence as he pulled his Beretta 38 from his shoulder bag. A short way on, we came to a clearing and a ramshackle structure of lashed poles topped by an orange tarp large enough to shield a dozen men. Tibaldi reached under a table and pulled out a box filled with supplies: sugar, flour, coffee, utensils. "They've run from us," he said. All was silent, except for the yelping of a pair of toucans in the treetops. The day was growing long, rain clouds were building in the east, and no one wanted to be caught here with darkness falling.