BETWEEN 1979 and 1985, some 40,000 people were murdered by right-wing death squads in El Salvador. Roberto d'Aubuisson, who died Thursday of cancer, was the most notorious symbol of those death squads, the offspring of a bloody alliance between far-right military officers and one of the hemisphere's most reactionary oligarchies.

While d'Aubuisson was the product of the complex, highly polarized Salvadoran society, he was also the creation of the Reagan administration and other conservative Republicans, who not only tolerated the bloodshed he espoused and led, but may have encouraged it.

While El Salvador moves toward peace, the history of the U.S. involvement in the bloody 12-year civil war is already being rewritten or forgotten. The war left 70,000 people dead, destroyed the nation and drew the United States into one of its longest Cold War paramilitary and military assistance efforts.

Before the war is forgotten, the arrogance and miscalculation that led to the U.S. policy that condoned d'Aubuisson and what he represented should be understood, in hopes it won't be repeated in the post-Cold War era now unfolding. Seemingly holding out the promise of more peaceful times, the new global arrangements may at some point sorely tempt a future U.S. administration into taking sides in -- or even fomenting -- foreign civil wars. But the bitter lesson of El Salvador is that ideological fear and a futile attempt to define the country in black-and-white terms led to tragedy for tens of thousands of people.

D'Aubuisson, who despite being cashiered from the army in 1979 was known throughout El Salvador as "The Major" because of his colorful military career, did not singlehandedly build the death squads. But he became their unmistakable symbol: His mixture of charisma, political ability and willing facility as spokesman for the alliance that drove and funded the death squads propelled him to a fearful prominence in his country and the region.

D'Aubuisson thrived because American leaders who feared a communist takeover in El Salvador deliberately turned a blind eye to the use of state-sanctioned terrorism against Marxist guerrillas, their supporters and suspected sympathizers. The responsibility for this policy began with President Reagan, and included Vice President Bush and U.N. Ambassador Jeane Kirkpatrick, together with Sen. Jesse Helms (R-N.C.) and Lt. Col. Oliver North, at the time an obscure aide on the National Security Council.

In a series of interviews with me several years ago, a number of former Reagan administration officials related how this policy emerged. They said that in the weeks between Reagan's November 1980 victory over Jimmy Carter and his taking office on Jan. 20, 1981, Reagan's transition team and supporters in Congress -- especially advisers to Helms -- emphasized to whoever would listen that the new administration was more interested in fighting communists in Central America than worrying about human rights there.

Helms has vehemently denied that he or his staff ever made such representations. But the fact is that from November 1980 through January 1981, some of the most notorious death squad killings took place. Human rights organizations with good information sources in El Salvador pointed immediately at d'Aubuisson. The killings included five respected leftist politicians on Nov. 27; the rape and murder of three American nuns and a lay worker on Dec. 4; and the murder of two American land reform advisers and the head of the land reform program, hated by the oligarchy, on Jan. 4.

Archbishop Oscar Arnuflo Romero had been killed in March 1980, at the hands of a d'Aubuisson protege, and, according to two men who participated in death squad activities at that time, with d'Aubuisson's knowledge.

The U.S. Embassy in San Salvador later said there was clear evidence of d'Aubuisson's involvement available to Washington at the time of the killings. But the Reagan administration made no overt move to threaten an aid cutoff or take other steps to curb the right-wing violence.

On national television, d'Aubuisson, using stolen military intelligence files, would denounce teachers, labor leaders, union organizers and politicians. Within days, their mutilated bodies would be found at one of the numerous, gruesome dumping grounds in the country. The Reagan administration claimed not to be able to figure out who was behind the killings.

But in the subsequent interviews I conducted, former administration officials with access to the intelligence data at the time told me that by 1982 most of the leaders of the far-right death squads had been identified by Washington as officers in the Salvadoran security forces and military, with direct ties to d'Aubuisson.

Yet only when U.S. public outrage at the bloodshed threatened to move Congress to cut off aid to El Salvador did the Reagan administration finally take the first step against d'Aubuisson and his network.

On Dec. 11, 1983, Vice President Bush met with Alvaro Magana, El Salvador's provisional president, and told him that unless the right-wing slaughter stopped, U.S. aid would be jeopardized.

According to a senior participant in the meeting, North, accompanying Bush, slipped Magana a piece of paper with no letterhead or watermark to make it traceable. The paper had the names of eight military officers and one civilian -- all close d'Aubuisson associates -- identified as death squad leaders.

None was ever brought to trial, but all either went into gilded exile or dropped out of sight. The death squad killings dropped sharply.

While Bush did not directly face down the death squads as he claimed in his 1988 presidential campaign, the meeting showed two things very clearly: First, the United States knew who the killers were; and second, when real pressure was finally brought to bear, the killing could be slowed if not stopped.

Such pressure could have been applied years -- and thousands of lives -- earlier, without jeopardizing the U.S. goals of defeating the guerrillas in El Salvador.

One man who worked with d'Aubuisson told me how the major sent him to be trained in interrogation techniques by veterans of Argentina's dirty war. The most effective technique, he said, was strapping a prisoner naked to a metal bed frame set in a pool of water several inches deep. Electric current would then be run to the frame, with a special wire to the genitals.

"That was the best," he said. "Not even the toughest could resist that."

That is the type of ally the United States chose in its fight against Central American communism. The tragedy is not just the bloody career of Roberto d'Aubuisson, but the policy that encouraged it. As Salvadorans celebrate the dawning of peace, the United States should not forget its share of the responsibility for the carnage in El Salvador.

Douglas Farah is The Washington Posts's special correspondent in Bogota, Colombia, and covered El Salvador from 1985 to 1990.