GUATEMALA CITY — A Guatemalan court on Friday found Gen. Efraín Ríos Montt, the former dictator who ruled Guatemala during one of the bloodiest periods of its long civil war, guilty of genocide and crimes against humanity.

Judge Yasmín Barrios sentenced General Ríos Montt, 86, to 80 years in prison. His co-defendant, José Mauricio Rodríguez Sánchez, who served as the director of intelligence under the general, was acquitted of the same two charges.

“We are completely convinced of the intent to destroy the Ixil ethnic group,” Judge Barrios said as she read the hourlong summary of the ruling by the three-judge panel. Over five weeks, the tribunal heard more than 100 witnesses, including psychologists, military experts and Maya Ixil Indian survivors who told how General Ríos Montt’s soldiers had killed their families and wiped out their villages....