A few footsteps from the vast, peaceful waters of the Tapajós River, far beneath the patchwork canopy of the untamed jungle, Juarez Saw Munduruku hacked at the dense bracken with his 18-inch hunting knife.

The 55-year-old leader of the Munduruku, an Amazon tribe, was marking its territory. For centuries the clan, known as the “red ants” for their deadly mass assaults on rival tribes and colonialists alike, would do so by placing the severed heads of their adversaries on spikes. Now, his men hammer handmade wooden signs with words in their native language stenciled in red paint onto tree trunks.

The meaning, however, is still unmistakable: This is Munduruku land.

The signs are man-made specks in an ocean of nature but are harbingers of a battle to come. It is a fight between the Munduruku, who have long sanctified this river, and Brazil’s government, which plans to flood much of this land to build a $9.9 billion hydroelectric dam, the São Luiz do Tapajós. The dam is one of seven planned for this river and part of a wider strategy across the Amazon that the energy ministry says is necessary to sate the country’s growing need for power. But the Munduruku say they have a constitutional right to remain on their territory — and that the government is refusing to acknowledge it, in violation of the law.

The battle echoes the government’s fight with indigenous communities on the Xingu River, another major Amazon River tributary, over the Belo Monte Dam. That project, first conceived in 1975, is now nearing completion. But environmental activists say that while the tribes there were ultimately divided — and defeated — by threats and bribery, the Munduruku have the determination and unity to take on the government and win.

The red signs are a start. They are part of a broader plan to protect the land that the Munduruku, who number 13,000 and live in a series of villages on the river, say they will, if necessary, defend to the death.

“We will fight to the end,” said Juarez Saw Munduruku. “This is our struggle. … I would die defending my land so that another generation can live here.”