Maj. Roger Miranda Bengoechea, a former top aide to Nicaraguan Defense Minister Humberto Ortega, had what one former colleague called a "correct revolutionary trajectory."

He studied economics in Chile during the leftist regime of Salvador Allende, fought as a member of the Sandinista Front in the successful revolution against Nicaraguan dictator Anastasio Somoza and for five years headed the Sandinista Defense Ministry's administrative arm, formally known as the secretariat.

But on Oct. 25, Miranda hitched a ride to Mexico on the Sandinista presidential plane and defected to the United States -- a move that Humberto Ortega has called "the most important betrayal" in the seven-year history of the Popular Sandinista Army.

In his first interview since defecting, Miranda said last week that his disillusionment with Marxism -- "I believed blindly in those ideas" -- evolved over a period of years as his ideals "kept bumping up against reality."

"It was a very difficult decision for me," Miranda said. "I know for {the Sandinistas} it was a great surprise. It's as if a link in a chain broke that no one expected."

In an emotional news conference last month, Humberto Ortega, brother of Nicaraguan President Daniel Ortega, confirmed that Miranda, whom he called alternately the "little worm" and "traitor Miranda," had been one of his closest aides. He said Miranda had access to "important military information and documents" and had made off with copies of some records.

He charged that the Reagan administration will attempt to "manipulate" Miranda's information and mount a "propaganda show" against the Sandinistas to undermine the peace process now under way in Central America.

In the interview, Miranda alleged, among other things, that acting on Humberto Ortega's direction he oversaw the diversion of $1.4 million in Defense Ministry funds to a numbered Swiss bank account for Ortega's personal use.

U.S. officials consider Miranda, who has undergone extensive debriefings by the Central Intelligence Agency, to be credible and to represent a major breakthrough in penetrating the Sandinista hierarchy.

As head of the secretariat since 1982, Miranda said he had access to minutes of closed meetings of the nine-member National Directorate of the Sandinista Front, which rules Nicaragua. He said he also acted as a liaison with the army general staff and other branches of the government and that he sometimes sat in on private conversations between the two Ortega brothers.

Miranda was interviewed in a guarded State Department conference room on the same day that Soviet leader Mikhail Gorbachev and President Reagan were discussing easing military tensions in Nicaragua at the White House. During more than four hours of questioning, arranged and monitored by State Department officials, Miranda alleged:That top Sandinista leaders have adopted a secret strategy to turn the regional peace plan signed by Nicaragua and four other Central American countries into a "weapon" to consolidate Sandinista control over Nicaragua and to eliminate the U.S.-backed rebels, known as contras. That secret military agreements have been negotiated with the Soviet Union and Cuba that call for a major military buildup over the next seven years, including the delivery of MiG21 jet fighters and enough arms for a Sandinista military force of 500,000 full- and part-time soldiers; That in addition to Humberto Ortega's Swiss bank account, top military officers have used a slush fund set up in Panama for personal expenses, and that aides to Interior Minister Tomas Borge allegedly accepted bribes from drug traffickers; That contrary to the peace plan, the Sandinistas continue to support leftist Salvadoran guerrillas and recently trained 15 of their men to use Soviet-made portable antiaircraft missiles; That in the event of an expected U.S. invasion of Nicaragua, the Sandinistas' secret defense plan calls for taking U.S. Embassy officials hostage and spreading the conflict throughout the region, including dropping bombs on Costa Rican targets; That the effects of the six-year contra war have begun to take their toll inside Nicaragua, and that the contras' greatest failure has been their inability to tap the discontent in Nicaragua's urban centers.

"As the years pass," Miranda said, "the situation of the Sandinista leadership gets more difficult. That does not mean that in the next month or so they will be defeated, but that the effects of the war are beginning to be felt in such a poor country as Nicaragua. Humberto Ortega has said he is beginning to feel the rope around his neck."

Queried Friday about Miranda's allegations, Maj. Rosa Pasos, who said her comments had been authorized by the defense minister, said, "We believe ex-major Miranda has a problem of emotional maladjustment."

Pasos added that Sandinista officials had questioned Miranda's mother after his defection and quoted the mother as saying that she fears her son "might even commit suicide."

During the interview, Miranda, 34, remained composed and answered questions in a calm, low-key manner. Dressed in a blue business suit, he appeared to choose his words carefully and said he had no information on some subjects raised by the reporters. He denied allegations by the Sandinistas that he spied for the CIA before he defected. "Before I left on Oct. 25, I had no contact with the CIA," he said.

Sometimes his comments, particularly on the importance of the contras, appeared to mirror administration views. Some key aspects of his story could not be independently verified.

Miranda's account was provided to reporters at a time when the administration is engaged in an uphill battle to win congressional approval for new contra aid and involved in a dispute with congressional Democrats over the peace pact signed by Nicaragua and four other Central American countries in Guatemala on Aug. 7.

Democrats charge that the administration is still intent on continuing the contra war at the expense of the peace process, while some administration officials argue that the Guatemala accords do not provide enough safeguards to ensure democracy in Nicaragua and stem what they see as the Sandinista threat of subversion in the hemisphere.

The administration last week asked the Senate to approve $22.8 million in what it calls interim "nonlethal" aid to last the contras until February, but the Senate agreed to provide only $9 million.

Democratic congressional aides noted on Friday that Miranda's interview with reporters was arranged by aides to Assistant Secretary of State Elliott Abrams, a chief proponent of the contra program. Additional press interviews are scheduled this week.

"I'm sure the people eager to continue the war will find {Miranda} useful," said an aide to House Speaker Jim Wright (D-Tex.), a strong backer of the peace process. "But I don't think it will have any significant impact."

Democratic aides said that the Soviet role in Nicaragua is already well known, and it is not surprising that the Sandinistas want a greater Soviet military commitment. "No doubt he has some juicy details," said one aide with long experience on Central America policy. "But it confirms what we know."

These aides said that more important than Miranda's allegations are Gorbachev's apparent support for the peace process and his possible willingness to reach an agreement with the United States on reducing the Soviet military commitment to Nicaragua. The Soviet leader reportedly told Reagan that any arms cutback is contingent on the continuation of the Central American peace process and a halt of U.S. aid to the contras.

"Whatever military accord this major brought out is obviously subject to relations between the United States and the Soviet Union," said the Wright aide. "Russia is not going to reach an agreement with us and then foul that because of Nicaragua."

Miranda described the Sandinistas as being insecure about their relationship with the Soviets and "afraid that the Soviets will negotiate over their heads."

The Sandinistas' continued pressure on the Soviets to step up military aid is in part an attempt to extract a deeper security commitment, Miranda said. In particular, he said, Nicaragua's rulers see their request for a squadron of MiG21s, which the Soviets had promised but never delivered, as a test of the Soviets' true intentions.

"The MiGs are of no use for guerrilla warfare," Miranda said. "But because the United States says they will not permit them {in Nicaragua}, if the Soviet government permits them, it means the Soviets are saying to the United States, 'We are taking responsibility for the Sandinistas.' What they've wanted is an umbrella of protection. Sometimes the Soviets have wobbled a little bit on that."

Miranda said that among the documents he brought with him were copies of two Nicaraguan-Soviet military accords negotiated in Managua by officials of the two countries and Cuba. Since the Sandinistas came to power, the three countries have negotiated two five-year military plans, Miranda said. The second plan was intended to cover 1986 through 1990, he said, but by the end of this year the contra war had forced the Sandinistas to use almost all the arms and other military aid supplied free by the Soviets.

In October, shortly before Miranda defected, officials met in Managua to revise the second plan to make up for the shortfall and develop a preliminary five-year plan for 1991 through 1995, which is based on the projection of defeating the contras by 1990.

He said the procedure for each plan has been for officials from the three countries to tentatively agree on the plans in Managua and then send them to Havana for review by Cuban deputy leader Raul Castro and the head of the Soviet military mission there before they go to Moscow for final approval. Miranda said few changes are made after they are drafted in Managua.

He said Cuba serves as a conduit for all Soviet weapons sent to Nicaragua and that the Soviets and Cubans also must approve the Sandinista shipments of Soviet arms to Central American guerrilla groups.

For example, he said, Cuban and Soviet officials were expected to discuss the Salvadoran rebels' request for SA7s, antiaircraft missiles, at a meeting scheduled for mid-November in Havana. He said although the Nicaraguans had recently trained guerrillas of the Farabundo Marti National Liberation Front to use the missiles, Humberto Ortega preferred that the Cubans hand them over.

Miranda said the Sandinista leaders were taken by surprise when the Central American presidents reached agreement on the peace plan. He said that after President Daniel Ortega returned from Guatemala, Miranda attended a meeting of the Sandinista Assembly, a group of top- and middle-level party officials, where the peace plan was described as "a weapon" the party should use "to strengthen the political plan of the revolution and eliminate" the contras.

Since they took power, Sandinista leaders have anticipated U.S. military intervention in Nicaragua, and the 1983 U.S. invasion of Grenada strengthened this fear and convinced them that they could not defeat the United States on the battlefield.

The Nicaraguan military drew up a revised contingency plan that calls for tying down U.S. forces in a guerrilla war while Sandinista leaders attempt to build international diplomatic pressure to force a U.S. withdrawal. The plan also provides for the Sandinista forces to create disorders in neighboring countries; for example, assisting Salvadoran guerrillas and bombing targets in Costa Rica.

Miranda also described in detail how some of the U.S. dollars allocated to the Defense Ministry ended up in secret bank accounts set up for Humberto Ortega and other high-ranking officers at the Panamanian branch of the National Bank of Paris. He said one account was opened in the name of a front company called Suplidora de Negocios S.A. to purchase home appliances and other luxury items for military officers and their families. Another company, which he said was called Impex S.A., was set up for Ortega's account.

Miranda said about $1.4 million was placed in that account between 1981 and 1985. In recent years, he said, Nicaragua's hard currency reserves have been very low, and few dollars were available to be diverted to any of the secret accounts.

When the political situation in Panama became unstable in 1985, Miranda said, the money in Humberto Ortega's account was moved to a branch of the same bank in Switzerland in the name of Representaciones Multilaterales del Caribe S.A. He said the Swiss account number was 58946. He said when he left Nicaragua the account had grown to $1,494,596.

There are corporate records on file in Panama for two of the three companies he mentioned, but it could not be immediately determined whether they are tied to Humberto Ortega or other Sandinista officials.

Miranda, who is living in an undisclosed location under U.S. protection, admitted that he also had benefited from privileges enjoyed by high-ranking military officials. He said all his economic needs were taken care of, including free use of a house and car with chauffeur.

He said he felt badly about the privileges, and they partly contributed to the "long process" of deciding to defect. He added that what he saw on official visits to communist countries had shaken his belief in Marxism. He said that when he began to have doubts, he visited war zones for the first time.

"I went looking for these mercenaries . . . . What I found was humble peasant families that had risen up in arms because of all the errors the Sandinista government had committed."

A few months after he visited the war zones, Miranda flew out of Nicaragua aboard the Sandinista version of Air Force One, named "The 19th of July," in honor of the day Somoza was ousted.

Staff researcher Michelle Hall contributed to this report.