In December, 1981, the CIA informed congressional oversight committees that it had begun building a highly trained commando force of 500 Latins to strike at targets in Nicaragua. Sixteen months later, this force has swelled to an army of 7,000 Nicaraguan men with ambitious political goals and uncertain U.S. control.

Members of the House and Senate Intelligence committees said in interviews that growing concern about the size of this CIA-supported army, its objectives and the question of control over it were major factors in their decisions last week to put brakes on the "secret war" in Central America.

Information now available from a variety of sources, viewed with the benefit of hindsight, raises questions about the candor of the CIA briefings for members of the Intelligence committees. Nevertheless, most of the lawmakers interviewed said they still believe they were informed accurately about details of the operation at every step.

The central problem for many of them, they said, was the growing contradiction between the limited objectives that Reagan administration officials stated for the covert operation in a dozen secret briefings on Capitol Hill and the ceaseless, sometimes startling growth of the insurgent force and the shifting focus of its activity from one month to the next.

"There is no question that the numbers increased far beyond what the committee anticipated," said Rep. William F. Goodling (R-Pa.). "I think as the force increases and diversifies, controlling it would be an impossibility."

Rep. Lee H. Hamilton (D-Ind.) said, "The committee kept track of it pretty well, but it got out of hand." Once this happened, he said, "there were great restraints on the capability of the committee to turn it around."

"What was particularly difficult for Congress," said Sen. Daniel Patrick Moynihan (D-N.Y.), "was that the definition kept changing of what the objectives were, and when the president proclaimed these people to be 'freedom fighters' there was an unmistakable sense that we were not fully apprised of the purposes."

Initially, administration officials characterized the missions of the secret army as the interdiction of arms traffic through Nicaragua to lefist rebels in El Salvador and the exertion of pressure to force the leftist Sandinista leadership of Nicaragua to "look inward" rather than exporting revolution, according to participants in the congressional briefings. Additional objectives, added months later, were to pressure the Sandinistas to be more democratic and to go to the negotiating table.

Despite President Reagan's reference last Wednesday to the CIA-supported anti-Sandinista guerrillas as "freedom fighters," his administration did not suggest in briefings for Congress that the secret army's real purpose was to bring down the Nicaraguan government.

Increasingly, though, the very size of the secret army, the intensity of its attacks inside Nicaragua and explicit statements by its leaders appeared to outpace the limited purposes outlined to Congress.

By the administration's figures, the 7,000 U.S.-backed Nicaraguan guerrillas now outnumber the 6,000 communist-backed guerrillas whose threat to the government of nearby El Salvador was the original justification for the CIA effort. In meetings with congressmen and senators, CIA Director William J. Casey has refused to set any limit on the ultimate size of the force, made up of Nicaraguan exiles of various factions and native Miskito Indians.

In the last week, the House Intelligence Committee voted to ban covert actions in Nicaragua, the Senate committee voted to permit continuation of the actions for a limited time subject to legislative approval, and Reagan stepped up his appeals for public support of the Nicaraguan insurgents.

Taken together, these events represent the most serious struggle between the executive branch and the congressional committees overseeing the intelligence agencies since the committees were established as permanent arms of the two houses in 1976 and 1977.

The congressional oversight machinery was created to establish, under law, the authority of the legislative branch of an open and democratic government to monitor executive activities that are secret, sensitive and have the potential for major international repercussions. As pioneers in an area where the legislative bodies of most other nations do not tread, the congressional committees operate in a twilight zone, where both sides are still feeling their way.

Unless a consensus can be formed in the coming weeks and months, the struggle over undercover action in Central America could bring about an even more serious crisis between Congress and the Reagan White House. Should the administration persist in backing the insurgents, against increasingly explicit opposition in Congress, the stage would be set for a battle of constitutional proportions involving war and peace, and the power to commit the United States to the use of force abroad.

As representatives and senators sketched the history of their involvement, the secret operations in Central America seemed at the beginning to be hardly big or tangible enough to merit concern.

In early March, 1981, within six weeks of Reagan's inauguration, CIA Director Casey brought the Intelligence committees a presidential "finding" that secret operations in Central America were important to U.S. national security. Such a presidential finding is required by a 1974 law. Under a 1980 law, it must be reported in a timely fashion to the two committees.

The initial Reagan administration program was outlined to the committees in very general terms, centering on the protection of the Salvadoran government from the communist-supported insurgency there. Casey also portrayed the program as resulting from inquiries from neighboring countries, such as Honduras and Costa Rica, about help against the spread of revolution.

The administration's emphasis was on undercover political and propaganda efforts and improved collection of intelligence about outside direction and arms for the Salvadoran rebels. An internal administration document of April, 1982, also spoke of the "9 March 1981 Presidential Finding on Central America" as an effort to interdict arms.

Despite the relatively vague nature of the finding and the proposed activity, some in Congress were concerned enough to dispatch personal letters of caution to the administration because of worries that, once begun, these activities could take on a life of their own.

For the new administration, 1981 was a year of deepening concern about Central America and high-level conflicts over what to do. The insurgency in El Salvador continued apace and, by the end of October, the State Department had failed in efforts to negotiate a cutoff of Nicaraguan support for the Salvadoran rebels.

Some officials, led by then-Secretary of State Alexander M. Haig Jr., favored a naval quarantine of Cuba and Nicaragua, but the Pentagon was leery. As the result of a National Security Council meeting on Nov. 16, 1981, Reagan approved a 10-point program including economic and military aid to friendly nations, U.S. contingency planning and military preparedness--but no U.S. military action.

One of the 10 points, according to NSC records, was to "work with foreign governments as appropriate" to conduct political and paramilitary operations "against the Cuban presence and Cuban-Sandinista support infrastructure in Nicaragua and elsewhere in Central America."

An accompanying document explained that this initially would involve a $19 million program to build a 500-man force, but that "more funds and manpower will be needed."

The document added:

"Covert activities under the CIA proposal would be intended to:

" Build popular support in Central America and Nicaragua for an opposition front that would be nationalistic, anti-Cuban and anti-Somoza. Gen. Anastasio Somoza, assassinated in 1980, was the Nicaraguan president overthrown by the Sandinistas.

" Support the opposition front through formation and training of action teams to collect intelligence and engage in paramilitary and political operations in Nicaragua and elsewhere.

" Work primarily through non-Americans to achieve the foregoing, but in some circumstances CIA might (possibly using U.S. personnel) take unilateral paramilitary action against special Cuban targets."

A few days later, on Dec. 1, Reagan signed the required "finding" that this new undercover effort in Central America was in the national interest. Shortly thereafter, in accordance with the law, Casey went to Capitol Hill to inform the two oversight panels of the presidential decision. There is no requirement under the law that he obtain their approval.

The CIA director spoke of the planned 500-man force as a carefully limited group whose target was the Cuban support structure in Nicaragua. No Americans and no mercenaries were to be involved, and no economic targets such as dams and power facilities were to be attacked.

The impression left with some members of the Intelligence committees was of crack teams of commandos hitting arms caches, ammunition dumps, Cuban military patrols and a couple of key bridges along the arms supply route in the dead of night and withdrawing unseen from Nicaragua to their Honduran bases.

Despite Casey's relatively low-key approach, lawmakers immediately recognized the plan as a serious advance in U.S. undercover activity. In the House committee room, there was almost a visible jolt, followed by a profusion of questions the CIA chief found difficult to answer.

What happens if you get caught, Casey was asked. What if the Nicaraguans enter Honduran territory in pursuit of the commandos? What happens if the beleaguered Nicaraguans ask for Cuban troops to defend their territory?

A Republican member said it was obvious that Casey had not thought through all the potential repercussions. A Democratic member was concerned even at that early stage about the legality, under the 1947 Rio Pact of hemispheric cooperation, of what the United States planned to do.

The reaction was not as strong in the Senate committee, according to participants, but concern was expressed there about the ultimate direction of the new program.

The CIA director presented the operation as one already under way. He mentioned at one point, in almost off-hand fashion, according to participants, that Argentines already had set up training camps for Nicaraguan exiles inside Honduras. In effect, the United States would be "buying in" to an existing operation, he was quoted as saying.

Casey's briefing in December, as participants recalled it, did nothing to suggest an anti-Sandinista political dimension, despite the discussion in the November NSC records of a broad opposition front backed by paramilitary action.

Casey returned to the congressional committees in February, 1982, and briefed the members, who had said they wanted to be closely informed on the progress of the operation. The meeting with House members was not particularly eventful, participants recalled, until the CIA's Latin America director, Dewey Clarridge, was asked how many commandos had been trained and replied that the force stood at 1,000 men.

To those who had thought of the force as 500 men, this was a disturbing revelation. CIA officials insisted they had informed the committee that the 500-man force did not include an additional 1,000 Miskito Indians who were undergoing training as commandos.

Records from the Nov. 16, 1981, NSC meeting reflect the administration's knowledge at the time that "The Argentines are already training over 1,000 men."

The oversight committees did not meet to review the program again until May, 1982. In the interim, newspaper stories revealed the existence of the CIA paramilitary program and President Reagan's approval to strike at targets inside Nicaragua.

News reports from Nicaragua on March 14, 1982, also revealed that two major bridges near the Honduran border had been blown up by saboteurs. The protesting Nicaraguan government immediately attributed the destruction to Reagan's reported covert operation and declared a state of emergency that is still in effect.

Casey, speaking in a different context the day before the bridges were blown, told a student group in Washington: "It is much easier and much less expensive to support an insurgency than it is for us and our friends to resist one. It takes relatively few people and little support to disrupt the internal peace and economic stability of a small country."

CIA officials confirmed to the House Intelligence Committee in May, 1982, that the key bridges had been blown up by a CIA-trained and -equipped demolition team. This confirmation brought no objection from the committee because the bridges were seen as supporting illicit arms traffic from Nicaragua to guerrillas in El Salvador, according to House committee members. "We had to do that," one member said.

Committee members questioned the CIA officials at length about the arms they had interdicted by this time and about whether they had discovered any Cuban military patrols, which they expected to find in the Nicaraguan countryside. The CIA officials said they had not actually captured or blown up any caches of arms or ammunition but that the presence of the paramilitary teams in the arms-trafficking corridors was dramatically reducing the arms flow to El Salvador.

The CIA officials reported that the force stood at about 1,100 men and that training was going well. No Cuban units, however, had been sighted, they reportedly said.

Over the summer of 1982, a decision was made to move the camps of the insurgents from Honduras, where there was increasing uneasiness among civilian officials, across the border into Nicaragua. House members, who were concerned about potential trouble for Honduras, were relieved to hear of this decision when informed in an August briefing.

The number of U.S.-supported insurgents had risen to nearly 1,500, according to the briefing. They were being outfitted with U.S.-financed equipment through Honduran military depots and were paid a subsistence fee of $23 per month, according to CIA officials.

The steadily growing size and public prominence of the secret war brought a reaction from Congress. In August, a conference of the Senate and House committees amended the secret intelligence authorization bill so as to limit the purpose of the CIA effort.

In language made public and enacted anew as "the Boland amendment" last December, Congress declared that no funds could be spent to support irregular activities "for the purpose of overthrowing the government of Nicaragua or provoking a military exchange between Nicaragua and Honduras."

In light of the congressional concern, heightened by a Nov. 8 Newsweek cover story on the secret war, last December's briefing was a shocker. Suddenly the number of U.S.-supported insurgents had jumped to 4,000, nearly three times as many as four months before.

This news closely followed public statements by Nicaraguan exile leaders associated with the CIA effort that their objective was to overthrow the Sandinista government.

In a closed-door meeting on Capitol Hill, Casey said the numbers had swelled because Nicaraguans were "recruiting themselves" to join the fight against the unpopular Sandinista regime.

Under close questioning, one of Casey's aides admitted for the first time that "command and control problems" had been encountered. He attributed these to the withdrawal of Argentine advisers because of the war with Britain over the Falkland Islands and declared that "firm control" over the operation had been reestablished.

The operation also had been forced to employ more ex-Somoza Nicaraguan National Guardsmen than had been planned, lawmakers were told, because they were the only ones who wanted to fight.

One result of the redoubled concern on Capitol Hill was enactment in public session of the Boland amendment. Another was a request to the CIA for summaries of the secret operation at least once a month from then on.

By the first week of February, lawmakers were informed that the ranks of the U.S.-supported warriors had swelled to 5,500. There had been a discernible shift in their targets--a ranch had been hit, a granary burned--and in the avowed objectives, which now included pressure to bring the Sandinistas to the negotiating table.

There was a stormy meeting of the House Intelligence Committee, with many members reportedly feeling they had been misled about the size and scope of the enterprise. The chairman of the subcommittee on oversight, Rep. Wyche Fowler Jr. (D-Ga.), announced that he planned an inspection trip to the region.

On the Senate side, similar concerns had prompted an inspection trip in January by Sen. Patrick J. Leahy (D-Vt.) and a bipartisan staff group. Neither fact-finding mission did anything to allay congressional concern. Both groups of travelers reportedly concluded that the Boland amendment was being violated in spirit if not in letter.

Beginning this March, argument increased between the committees and Casey over the nature and purposes of the covert operation and whether the Boland amendment was being violated. A flurry of publicity in late March and early April--including detailed accounts by Washington Post and Newsweek correspondents of their observations of the CIA-supported guerrillas as invited guests of the supposedly secret force--provoked consternation in Congress.

CIA and State Department officials, called to Capitol Hill to explain, denied they had approved the reporters' visits. In lawmakers' minds, this raised even more urgently the question of U.S. control, especially since the size of the force was moving up toward the most recent estimate of 7,000 men.

The administration, under fire, sent Secretary of State George P. Shultz to the House committee to augment Thomas O. Enders, assistant secretary of state for inter-American affairs, who had been present with CIA Director Casey in nearly all the previous briefings on Capitol Hill, representing the State Department. On April 26, President Reagan himself summoned several House members to an Oval Office meeting to plead for a continuation of the secret operation.

In the view of some lawmakers, Reagan's unusual speech to a joint session of Congress on April 27 was designed to win support for the secret war in Nicaragua as much as it was to gain approval for more military aid for El Salvador.

But the public exhortation and private pleas to members of Congress, including a telephone call by Reagan from Air Force One to Sen. Walter D. Huddleston (D-Ky.) last week, failed to stop a majority of both the House and Senate Intelligence committees from acting to have the covert operation curtailed or much more clearly defined.

"We want the president to tell us in plain language just what it is he wants to do relative to Nicaragua," Senate Intelligence Committee Chairman Barry Goldwater (R-Ariz.) said in explaining the vote in his committee on Friday.

To make certain this is done, the Senate unit voted to permit the undercover war to continue only through Sept. 30 without a new presidential finding that must satisfy a majority of the committee.

In Honduras yesterday, a spokesman for the CIA-backed guerrilla force told United Press International that this deadline is acceptable. "There's no problem," he said. "We'll be in Managua in five months."