MEXICO CITY — A Guatemalan judge on Monday ordered Efraín Rios Montt, the former dictator, and his intelligence chief to stand trial on charges of genocide and crimes against humanity in connection with the massacres of villagers in remote highlands three decades ago.

The ruling clears the way for a public trial for Mr. Rios Montt, a former general who ruled Guatemala for 17 months in 1982 and 1983 during the bloodiest period of the country’s long-running civil war. It is a stunning decision for Guatemala, where the military still wields significant power behind the scenes and the country’s elected governments have struggled to build democratic institutions....