SÃO PAULO, Brazil — Brazil has made great strides in recent years in slowing Amazon deforestation and showing the world it was serious about protecting the mammoth rain forest.

The rate of deforestation fell by 80 percent over the past six years, as the government carved out about 150 million acres for conservation — an area roughly the size of France — and used police raids and other tactics to crack down on illegal deforesters, according to both environmentalists and the government. Brazil’s former environment minister, Marina Silva, became an internationally respected defender of the Amazon. She ran for president in 2010 on the Green Party ticket and won 19.4 percent of the votes.

But since Dilma Rousseff was elected president in late 2010, there have been signs of a shift in the government’s attitude toward the Amazon. A provisional measure now allows the president to decrease the lands already created for conservation. The government is granting more flexibility for large infrastructure projects during the environmental licensing process. And a proposal would give Brazil’s Congress veto power over the recognition of indigenous territories.

“What is happening in Brazil is the biggest backsliding that we could ever imagine with regards to environmental policies,” said Ms. Silva, who now devotes her time to environmental advocacy.