He was a career U.S. diplomat of the old school: polite and professional, an intent listener who never gave offense. But when Harry G. Barnes Jr. presented his credentials in 1985 to Chile’s dictator, Augusto Pinochet, he threw down a quiet challenge that eventually helped nudge the general from power.

“The ills of democracy can only be cured with more democracy,” the new American ambassador told Pinochet. Several days later, in an interview, the military ruler spluttered an angry retort. “Since when are some ambassadors arbiters of our internal problems?” he demanded. “We are not anyone’s colony or slave.”

Mr. Barnes’s mission was to convince both Pinochet and his demoralized opponents that the Reagan administration would no longer tolerate the general’s abusive grip on power. For three years, Mr. Barnes pushed steadily for political change in Chile — opening his embassy to opposition leaders, standing up publicly for victims of repression and making it clear that he had Washington’s blessing.

In 1988, after 15 years of military rule, Pinochet was peacefully defeated in a national plebiscite and forced to step down as president in 1990, ushering in an era of democratic governance and economic success that has become a model for Latin America. Mr. Barnes left Chile soon after the plebiscite and retired from the Foreign Service. He died Aug. 9 in Lebanon, N.H., at 86. His family said he had contracted an infection.

During 38 years in the Foreign Service, Mr. Barnes was posted in eight countries, learning to speak many of their languages, and he served as ambassador to India (1981 to 1985) and Romania (1974 to 1977) as well as Chile. In India, he plunged into negotiations on nuclear and arms issues after a generation of ideological estrangement.

Harry G. Barnes, Jr., in New Delhi in 1981. (Associated Press)

After retirement, he worked for the Atlanta-based Carter Center as director of human rights and conflict resolution programs from 1994 to 2000, traveling often to global trouble spots.

But it was Mr. Barnes’s role in Chile that earned him a niche in history. A small country that loomed large in the ideological wars of the 1960s, Chile embarked on a chaotic socialist period under President Salvador Allende.

In 1973, Allende and Chile’s leftist aspirations were crushed by a military coup that was welcomed by the Nixon administration and reportedly abetted by the CIA. Pinochet, who was army chief, seized power and vowed to extirpate communism from Chilean soil. Over the next several years, thousands of political activists were detained, tortured or vanished in custody.

By the mid-1980s, the Reagan administration in Washington had soured on Pinochet and was looking for an antidote to its controversial anti-communist role in Central America and the Iran-contra scandal. Mr. Barnes was sent to Chile to push for a return to civilian rule. It was a delicate task, with powerful adversaries in both capitals, and he tackled it with characteristic and quiet determination.

“Harry had to navigate working with the opposition, the socialists and Christian Democrats, without becoming an enemy of the government or the Chilean right,” said Elliott Abrams, then the top State Department official for Latin America and now with the Council on Foreign Relations in New York. “He also had to navigate in Washington,” where the new policy on Chile was opposed by powerful conservatives such as the late Sen. Jesse Helms (R-N.C.). “I think he did a magnificent job.”

Mr. Barnes made few public pronouncements in Chile but many pointed gestures, such as participating in candlelight protest vigils. In 1986, he and his wife, Elizabeth, braved police tear gas to attend the funeral of Rodrigo Rojas de Negri, 19, a Chilean exile visiting from Washington who was burned during a protest in Santiago and dumped by a military patrol to die.

“If the United States was trying to be clear about our concern for human rights,” he said later, “the death had to be protested.”

Pinochet, infuriated by Mr. Barnes’s activities, barred the ambassador from his palace and ordered his image cropped from ceremonial news photos. But years later, with Pinochet an aging international pariah, Mr. Barnes was awarded Chile’s highest honor in recognition of his contribution to the restoration of democracy.

“He had a very hard time here in Chile,” Andres Zaldivar, a longtime Christian Democratic senator, wrote in a Chilean newspaper last week, noting that Mr. Barnes faced constant harassment from the regime but always “kept the embassy open to us in the opposition.” Without intervening in domestic politics, Zaldivar wrote, Mr. Barnes was “a very important factor in the democratic transition process.”

Harry George Barnes Jr. was born June 5, 1926, in St. Paul, Minn. After Army service, he graduated in 1949 from Amherst College in Massachusetts and joined the Foreign Service in 1950, pausing in mid-career to receive a master’s degree at Columbia University in 1968.

Besides his wife of 64 years, the former Elizabeth Sibley, survivors include three children, Douglas M. Barnes of Miami, Sibley A. Barnes of Sarajevo, Bosnia, and Pauline M. Barnes of Walpole, N.H.; a brother; and a grandson. A daughter, Adrienne Barnes, died in 2003.

Mr. Barnes made a mark during other postings beside Chile. In Nepal, he was remembered for commandeering a U.S. government plane to obtain rabies vaccines for some children who had been bitten by a dog.

In Romania, while he was deputy chief of the U.S. mission in the 1960s during the communist reign of Nicolae Ceausescu, it was discovered that Mr. Barnes’s voice was being picked up and broadcast from inside the embassy. In an oral history years later, he recalled a colleague handing him a note that said, “You’re on the air.”

After a little experimenting, the mystery was solved: A microphone had been planted in the heel of one of Mr. Barnes’s newly-repaired shoes.

“I had sent them out with our maid,” he recounted. When they came back, something seemed amiss. “One heel felt a little bit higher, so I sent them back. When they came back they were OK, but that of course gave a clue to as to where to look.”