For Angela Muruzapal de Westerkamp, the wife of an Argentine human rights activist and the mother of a political prisoner, the new era in United States diplomacy began on the humid night in February when her husband did not come home.

Jose Federico Westerkamp, an internationally renowed physicist who has helped lead the campaign against kidnapings and arbitrary detentions by Argentine government security forces, had been arrested in a latenight raid on the office of a Buenos Aires human rights organization. Armed plainclothes officers took away Westerkamp, seven associates and armloads of documents alleged to indicate violations of national security. The head of the country's Permanent Assembly for Human Rights, Emilio Mignone, was arrested about the same time at his apartment.

Until Adolfo Perez Esquivel won the Noble Peace Prize in October, Westerkamp and Mignone were the best-known human rights activists in Argentina. Neither had been arrested for their work before, but Westerkamp's wife knew precisely what to do. By the time the taxis had began clogging downtown Buenos Aires the next morning, she was sitting in a booth at the international telephone office, calling everybody she could think of -- from London to Washington -- who might help release her husband from jail.

By March 6, the government had released all the activists they had detained, pending "investigations." "It was obvious both to reporters and to the Mignones and Westerkamps that U.S. Ambassador Harry Shlaudeman had, as one observer delicately put it, "talked to certain people" to ensure the activists' freedom.

But there were no public statements from the embassy -- no confirmation, no denial, no criticism of the Argentine government for having permitted the arrests. In the deliberately low-key atmosphere of the following week, the Reagan administration's much-discussed "private diplomacy" -- quiet, behind-closed-doors negotiations rather than public pressure -- had been given what was assumed to be first real test here.

The Carter years are over in the authoritarian military governments of the southern part of South America. Relations with ruling generals -- Argentina's Roberto Viola and Chile's Augusto Pinochet -- are warmer than they have ever been. Viola recently completed a highly successful tour of Washington and New York, and envoys from Chile and Brazil also have visited Washington for less-publicized meetings with American military officials.

The United States, in turn, is sending its generals south with a frequency and friendly visibility unheard of during the Carter administration. fRetired general Vernon Walters' February swing through South America included Brazil Argentina and Chile and ended on notes of mutual praise. U.S. Army Gen. Edward Meyer recently completed a cordial week of talks and military ceremonies here, and U.S. Air Force Chief of Staff, Gen. Richard Ingram arrived here this week.

The changes wrought by this official good will already are visible in South America. President Reagan has asked Congress to repeal its embargo on arms sales to Argentina. The State Department has reversed a 1979 ban on Export-Import Bank credits for financing U.S. exports to Chile, and after a year of exclusion, Chile has been invited by the United States to join in naval exercise in the southern Atlantic.

And last month in Montevideo, Uruguay, Vice Adm. Hugo Marquez told reporters that after four years of refusing to sell arms, the United States had once again began selling weapons to the military-controlled Uruguayan government.

"The country just bought three [American] antisubmarine aircraft that are the most advanced type in Latin America," Marquez said. "I am very satisfied with the presence of Ronald Reagan in the United States government. He is a man with the right idea about things."

What does all this mean? Just how effective were the difficult, controversial years when the U.S. State Department publicly condemned political killings and torture in Latin America? Are the military governments in Chile, Argentina, Uruguay, Paraguay and Bolivia in for a tangibly different time now that the American diplomatic emphasis is on terrorism and "hemispheric security" rather than what we have traditionally defined as human rights?

"It's another style," Mignone said recently, during a conversation at Westerkamp's book-cluttered Buenos Aires apartment. "Maybe, just now, it's more effective for us."

"We have seen a real rise in repression since January that coincides with Reagan's administration," said a Chilean attorney in a recent interview in Santiago. "The increase in arrests in January and February is very high -- more than double the years before. That's a fact. Is it related to the American change?" He paused and then shrugged. "I don't know."

Although Mignone and Westerkamp discount any connection, outside observers such as Amnesty International and the Washington-based Council on Hemispheric Affairs have drawn attention to what they see as a disturbing trend in Argentina.In March, three men were abducted, apparently by security forces, and were reported missing. All three have since turned up alive.

Police also recently detained for several hours about 70 women from the Mothers of the Plaza de Mayo, a group of several hundred women who assemble here every Thursday to request information about children and grandchildren who disappeared during the government's "antisubversion" campaign of the late 1970s.

Just what happened between the United States and Latin America during the Carter administration will probably be passionately argued for some time. There are those who believe that all Carter got for his administration's occasionally strident efforts was anger and that the governments went right on arresting and torturing people.

The only visible results of the Carter era, these people argue, is the backlash. As examples they cite defiant windshield stickers that road, "Argentines are right -- and human," unprecedented Argentine grain sales to the Soviets as Arentine grain sales to the Soviets as Argentina pointedly ignored the U.S. request for an international boycott and armies and navies that reacted to American arms embargoes by spending vast quantities of money on other nations' arms.

"Since we're not selling equipment and training to them, they have looked elsewhere, and they're finding it," said an informed observer in Chile, where all U.S. arms sales or deliveries have been blocked since 1979.

In Argentina, where a 1978 congressional amendment blocked U.S. arms shipments, American observers believe the armed forces are heavily shopping the European markets for what one source called "pretty much a wholesale replacement of at least the surface forces."

United States manufacturers sold about $30 million worth of arms to Argentina during 1977 alone, according to Navy Capt. Walter Raymond Beck, commander of the U.S. military group in Buenos Aires. It is against that background that Reagan, following Viola's visit to Washington, asked Congress to lift the embargo.

Mignone and Westerkamp, like most other South American human rights activists, firmly believe Carter's policy did what it set out to do -- it saved dissidents' lives.

"I'm absolutely convinced that if this policy hadn't existed, the situation would have been much worse in Argentina," said Mignone. "Instead of killing thousands, they would have killed thousands more, because they would have done it the impunity. . . . This is a regime that is sensitive to international image."

"I think the prestige of the United States has gone up enormously because of all this [Carter pressure]," added Westerkemp. "You remember the violent, 'Yankee, go home,' demonstrations you used to see in Latin America? I don't think this sentiment exists any more."

"We can't cite specific cases where it saved lives," said the Chilean attorney, "but when Carter took the presidency and announced his policy, we began seeing a quantitive decline in the number of violations in Chile. . . . We had the dissolution of Dina [Chile's highly feared secret police force] in July 1977. There were no new disappearances after 1977."

"It was responsiveness to the reality," said a former Carter human rights official. "Quiet diplomacy was 98 percent of the Carter policy as well, so it isn't as though they're trying something new . . . There were, in quiet diplomacy, talks about torture, summary executions, disappearances, detentions without trial, the right of option, habeas corpus, due process, the rule of law, international law, international standards."

It was when that kind of diplomacy failed, the official said, that the public pressure was brought to bear on the governments.

It has been argued that the policy change has been somewhat overdramatized in Latin America -- that the Carter administration was already toning down its criticism of the military governments as the severity and extent of the repression eased. In chile, for example, diplomatic sources said the restrictions on Ex-Im Bank financing would almost certainly have been dropped anyway, since they were thought to be penalizing American exporters more than their potential Chilean customers.

To many of its supporters, the strongest legacy of the human rights policy is an international one. When Angela de Westerkamp sat down at the telephone office to begin trying to free her husband, she was calling a network of human rights activists that took its solid shape during the Carter years.

"It doesn't all stop with us," said the Carter official. "We are a powerful addition. If we withdraw for an interval, we become like an obstacle in the path of a river that isn't going to stop going."