Argentina's military leadership ordered the preparation of detailed plans for invading the Falkland Islands more than two months before the incidents in the South Atlantic that Argentina later said had provoked the conflict with Britain, according to Argentine military sources.

The sources said that the Argentine military government had adopted a general plan in September 1981 to take over the Falklands by diplomatic or military means. Using the detailed invasion plans prepared in January, the ruling junta decided to go ahead with the attack March 26, a week before the actual landing and during a time when intensive negotiations continued among Argentine, British and U.S. diplomats.

The night of the junta's decision, a detachment of Argentine Marines secretly landed on the disputed islands of South Georgia, where trouble had begun several days earlier over the raising of an Argentine flag, the sources said.

The new account by high-ranking Navy and Army sources indicates that the April 2 invasion was not entirely the hasty, impetuous action that has been portrayed here following Argentina's defeat by a British task force.

Military sources insist that the Argentine plans for military actions were activated only after the public dispute with Britain began over 39 Argentine workers on South Georgia.

Had the military command waited another six weeks to act, these sources said, Argentina would have been far better prepared to fight because of new imports of arms then expected from France and other countries.

Details about Argentina's role in the crisis are emerging as military officials conduct wide-ranging investigations of the Falklands conflict, which until now has been largely clouded here by the government's official silence and extensive propaganda.

The official studies are considered a key to the resolution of ongoing disputes within the armed forces that continue to pose a threat to Argentina's Army-ruled government, political leaders and analysts say.

Among the details revealed by several high-ranking officials in interviews here:

* While U.S. Secretary of State Alexander M. Haig Jr. was officially acting as a mediator between Argentina and Britain last April, U.S. officials secretly warned Argentine president and Army commander Leopoldo Galtieri that the United States would support Britain if the dispute was not resolved diplomatically. Galtieri also was warned that the British would not hesitate to attack Argentine forces to regain the islands and was given an American assessment that the British would win.

* Following two such warnings, delivered at a series of late-night meetings at Army headquarters, Galtieri appeared to associates to be convinced that Argentina should unilaterally withdraw its troops from the Falklands. But he later was swayed back to a hard-line stance by Navy Commander Jorge Anaya and other military leaders.

U.S. officials have refused to comment here on specific efforts by the Reagan administration to mediate the crisis. But Argentine military officials said the U.S. effort ultimately was doomed by the long tradition of Argentine nationalism over the claim to the Falklands, and by the long-held intention of the military to reoccupy them through whatever means necessary.

* As early as mid-April, less than two weeks after the invasion, leading staff officers of the Army openly opposed Argentina's policy of risking a military fight with Britain. At that time, the junta insisted publicly that it had the full support of the country.

* Argentina's Navy had only 16 operating A4 Skyhawk fighter-bombers at the beginning of the conflict, despite the dozens it was publicly credited with. The Argentine Air Force had other squadrons of the American-made planes, but of the Navy's 16, only four were still in service when Argentina surrendered.

The accounts provided by military officials here indicate that it was the Argentine Navy and its commander, Anaya, that pushed the Falklands invasion as a long-treasured project and that swung the balance in Argentina's decision to stick to its insistence on sovereignty over the islands.

Galtieri, who often struck a public pose as a gritty, unbending general, emerges in these new accounts as a malleable, somewhat indecisive figure who at times appeared to share the views of the last adviser he talked to in his characteristic all-night meetings.

Military officials said, however, that Galtieri became firmly committed to an eventual battle after the U.S. announcement of support for Britain and the first skirmishes around the Falklands in early May.

These actions, which included Argentina's sinking of the British destroyer Sheffield, led the junta members to believe Argentina could hold its own with the British task force and force a negotiated solution in the junta's favor.

Ironically, Galtieri was the first of the junta members to be forced out after Argentina's surrender June 14, while Anaya remains in his post and appears headed for a graceful retirement later this year.

Anaya was one of the architects of a 1975 invasion proposal that sat on the shelf until September 1981, when the government adopted a plan gradually to build up diplomatic pressure on Britain for concessions in the South Atlantic. The first round of negotiations between Argentine and British officials took place in New York in late February of this year.

Some military officials here say they were initially pleased with the results of the talks. Nonetheless, they say staff officers of the various services had been ordered in January to prepare the detailed plans for an Argentine invasion.

One high naval official, who like other officers insisted on remaining anonymous, said the planning consisted of deciding such issues as how many troops would be involved in an attack, where landings could be made and where Argentine forces would be assembled and embark.

This officer and other officials said a six-week delay would have given Argentina a better chance militarily. At the time, Argentina was expecting the delivery of more than six new Super Etendard fighter-bombers from France, along with a large supply of air-to-surface missiles from several sources.

As it was, Argentina had only five Super Etendards and five Exocet missiles during the conflict and the Navy faced severe problems because many of its A4 Skyhawks were out of service.

Military sources confirmed that one crucial factor in the junta's decision to go ahead with an invasion plan that it had rejected twice before was the perceived attitude of the Reagan administration, which was seen as an ally that would remain neutral in the crisis despite the U.S. warnings to the contrary.