BELEM, Brazil - A rancher goes to trial today in the killing of an American nun whose death while trying to save the Amazon rain forest now threatens to strip away the impunity of the region's often-violent elite.

Vitalmiro Bastos Moura is one of two ranchers accused of ordering the 2005 killing of Dorothy Stang, 73, a naturalized Brazilian originally from Dayton, Ohio.

She spent the last 23 years of her life in Anapu, a hardscrabble town on the edge of the Trans-Amazon Highway, where she helped build schools, and taught settlers to defend their rights and to respect the rain forest - earning the enmity of powerful men who hoped to exploit it.

She was slain by six bullets at close range on a muddy patch of road deep in Para state.

The gunman, his accomplice and an intermediary have been convicted in Stang's death, but Moura is the first alleged "mandante" - mastermind - to stand trial.

"If Moura is convicted, ranchers will think twice before ordering this kind of killing," said Jose Batista Afonso, a lawyer with the Roman Catholic Church's Land Pastoral, which defends the land rights of the poor.

Over the last 30 years, 1,237 rural workers, union leaders and activists have been killed in Brazilian land disputes. Of those killings, 772 took place in Para.

In all of Para's history, only four alleged "mandantes" have stood trial. All four were convicted. But not one remains behind bars.

Prosecutors allege Moura and another rancher offered the gunmen 50,000 reals - about $25,000 - to kill Stang over a patch of rain forest Stang wanted to preserve and the ranchers wanted cut down for pasture. No trial date has been set for the other rancher.

The case drew international attention and comparisons to the 1988 killing of environmental activist Chico Mendes. Shortly after the killing, President Luiz Inacio Lula da Silva ordered the army into the region, suspended logging permits and ordered large swaths of rain forest off-limits to development.

"The international attention to the case has forced the government to move quickly, which has been very good," said Tim Cahill of Amnesty International.

Stang's brother David, who planned to attend the trial with his twin brother Thomas, said he felt confident Moura would be convicted.

"I feel Brazil will do Dorothy justice," he said by phone from his home in Colorado. "This is not about revenge. This is about justice for the poor."