UNDER THE brief, bloody and brutal tenure of Gen. Efrain Rios Montt, one of Latin America’s most notorious dictators, Guatemala’s military government earned a global reputation for cruelty 30 years ago. With scorched-earth tactics to prosecute a war against leftist rebels, Guatemalan troops liquidated villages in the Mayan Indian highlands, murdering, raping and torturing unarmed men, women and children with impunity.

Until recently, that impunity extended to Mr. Rios Montt himself, who not only escaped punishment but served most of the past 20 years as a legislator; in 2003, he even sought to recapture the presidency. But in a stunning reversal of fortunes, the former strongman, having lost his seat in the legislature, was stripped of immunity and, in January, indicted for war crimes. Last month, a judge rejected his plea for amnesty.

Even by the blood-soaked standards of Central American Cold War conflicts, Guatemala’s three-decade-long civil war stood out for its carnage and depravity. More than 200,000 people died, many of them in massacres and most of them civilians at the hands of the government’s security forces.

The violence ran unchecked during Mr. Rios Montt’s presidency, which he seized, and lost 17 months later, in coups. While he has denied responsibility for what he called the “excesses” of government troops, he was the commander in chief, and, by his own account, a fully engaged one. As he put it to an American documentary filmmaker at the time, “If I can’t control the army, then what am I doing here?”

Mr. Rios Montt was pardoned by his successor, but under international law there can be no immunity from prosecution for genocide or crimes against humanity. A United Nations truth commission in Guatemala, established after the civil war ended in 1996, found that security forces, in targeting the indigenous Indian population, had committed genocide.

That Mr. Rios Montt, who is 85, may now stand trial is gratifying to victims’ relatives, hundreds of whom flocked to see prosecutors present charges in January. It is also a testament to the courage and determination of prosecutor general Claudia Paz y Paz. In office just over a year, Ms. Paz has vigorously pursued drug traffickers, human rights violators and organized crime. She is setting an example not just for her countrymen but for Latin America, where strongmen like Mr. Rios Montt have rarely faced justice.

Ms. Paz deserves cover from new president Otto Perez Molina, once a high-ranking military officer. Though the president has faced questions about his own actions in the civil war, he wants U.S. military aid to fight drug traffickers. Sensibly, Congress has conditioned any help on improvements in Guatemala’s traditionally toothless justice system and on an end to impunity. The prosecution of Mr. Rios Montt, if it goes forward, is an encouraging step in that direction.