MEXICO CITY — In 1999, President Bill Clinton went to Guatemala and apologized. Just two weeks earlier, a United Nations truth commission found Guatemalan security forces responsible for more than 90 percent of the human rights violations committed during the country’s long civil war.

Mr. Clinton’s apology was an admission that the Guatemalan military had not acted alone. American support for Guatemalan security forces that had engaged in “violent and widespread repression,” the president said, “was wrong.”

But that long history of United States support for Guatemala’s military, which began with a coup engineered by the Central Intelligence Agency in 1954, went unacknowledged during the genocide trial and conviction of the man most closely identified with the war’s brutality, the former dictator Gen. Efraín Ríos Montt.

During a month of testimony before the three-judge panel that found General Ríos Montt guilty last Friday, the prosecution never raised the issue of American military backing in the army’s war against leftist guerrillas. The 86-year-old former dictator barely mentioned the United States when he argued in his own defense that he had no operational command over the troops that massacred and terrorized the Maya-Ixil population during his rule in 1982 and 1983.