The basic conclusions of the Warren Commission have stood up against a spate of conspiracy theories that seemed to respond to an American need to make the assassination of the young president seem less random, less senseless. But the commission might have been less mystified about the probable motive had it not been for the CIA, which feared that a link might be established between the assassination and the agency's plots to kill Castro.

Since the commisssion filed its report, evidence has emerged--some of it still officially secret--suggesting a chain of circumstance that led Lee Harvey Oswald to become the self-appointed avenger of persistent efforts by the CIA, under presidents Eisenhower, Kennedy and Johnson to assassinate Fidel Castro.

Castro's agents penetrated many of the assassination plots. His informants circulated in the Cuban community in Miami, the main staging point. In 1978 Castro told a visiting House investigating committee, "We were constantly arresting people trained by the CIA . . . with explosives . . . with telescopic rifles."

On Sept. 7, 1963, Castro showed up unexpectedly at a Brazilian Embassy reception in Havana and launched into a tirade against President Kennedy and the CIA, accusing them of plotting his death. "Let Kennedy and his brother, Robert, watch out," he said. "They, too, could become targets of assassination."

After his impromptu speech at the Brazilian Embassy, Castro expanded his warning of retaliation in a long interview with an Associated Press correspondent, Daniel Harker.

In New Orleans, where Oswald was living, Harker's story appeared at the top of Page 7 of the Times-Picayune of Sept. 9. It started this way:

HAVANA (AP)--Prime Minister Fidel Castro said Saturday night "United States leaders" would be in danger if they helped in any attempt to do away with leaders of Cuba.

Bitterly denouncing what he called recent U.S.-prompted raids on Cuban territory, Castro said, "We are prepared to fight them and answer in kind. United States leaders should think that if they are aiding terrorist plans to eliminate Cuban leaders . . . they themselves will not be safe."

It is not established that Oswald read the story, but his wife, Marina, later said that he was an avid reader of newspapers, including the Times-Picayune.

The story came at a time when Oswald, an admirer of Castro, was in a state of agitation and frustration. He had lost his job. He had been arrested in a scuffle while distributing pro-Castro leaflets. He had engaged in an angry debate on the radio, saying, "Cuba is the only revolutionary country in the world today."

In the days after the publication of the Castro interview, events in Oswald's life appeared to take a decisive turn. On Sept. 23 he sent his wife and child to stay with their friend, Ruth Payne, in Irving, Texas. On Sept. 26 he traveled by bus to Mexico City, telling a passenger he wanted to go to Cuba and see Castro.

On Sept. 27 arriving in Mexico City, he went directly to the Cuban consulate to apply for a visa. Told that he could only get a transit visa-- he first needed a Soviet visa --he went to the Soviet Embassy, where he was turned down.

After shuttling back and forth between embassies--his telephone calls from the Soviet mission monitored by the CIA--he returned to the Cuban consulate on Oct. 1 with an insistent demand for permission to go to Cuba.

The consul, Eusebio Asque, finally threw him out, saying, "Instead of helping the Cuban revolution, you are really harming it."

What Oswald had said to produce this reaction was not known for some time. The CIA succeeded in getting Chief Justice Earl Warren to reject staff proposals to go to Mexico City and look into the Cuban connection. The agency was in desperate fear that the assassination might turn out to have been an act of Castro retaliation for its attempts to kill him.

Then, on June 17, 1964, FBI Director J. Edgar Hoover, who was conducting his own cover-up of bureau contacts with Oswald, sent a top-secret letter to J. Lee Rankin, chief counsel of the Warren Commission. As far as is known, the letter was never brought to the commission's attention, though it ended up in its voluminous files.

The letter cited statements made by Castro relating to the Kennedy assassination that the bureau had learned of "through a confidential source which has furnished reliable information in the past."

The substance of the letter remains classified to this day. But it has been learned that the informant was an American communist, working with the FBI, who had returned from a visit to Havana. As summarized in Hoover's letter, Castro said that "Oswald had vowed, in the presence of Cuban consulate officials, to assassinate the president."

Subsequently, a British correspondent, Comer Clark, quoted Castro as saying that Oswald had stated, "Someone ought to shoot that President Kennedy. Maybe I'll try to do it."

The Cuban consul clearly considered the threat a provocation. There is no reason to believe that he encouraged Oswald to act on it. But why didn't Castro warn the United States Government about the homicidal young man?

In 1964 Castro gave various explanations -- that he didn't take the reports from his embassy seriously, that he had no diplomatic relations with the United States, and that he suspected Oswald was part of some conspiracy to embroil him in an assassination attempt that might be used as a pretext for an invasion of Cuba.

But in 1978, interviewed in Havana by the House investigating committee headed by Rep. Louis Stokes, Castro denied prior knowledge of Oswald's plan. He said, "If Oswald would have done something like that, it would have been our duty to inform the United States."

Thus, after two decades, it appears that the Kennedy assassination may have involved a tragic and historic irony, a conspiracy of circumstances in which an arrow launched to kill a troublesome foreign leader fell back to slay our own.