On the eve of Argentina's April 2 invasion of the Falkland Islands, President Leopoldo Galtieri received three urgent phone calls from his then warm friends in the White House.

The first two calls, according to the story now told by high Argentine sources, Gen. Galtieri refused to answer. And by the time he agreed to speak to President Reagan, what has become the South Atlantic crisis was already under way.

That reported ducking by the Argentine junta leader of his newly acquired close partners is being cited here now as symptomatic of the fragility of that nascent U.S.-Argentine alliance that has been shattered by the conflict between Britain and Argentina.

Despite Galtieri's intention to reorient the traditionally neutralist Argentine foreign policy toward the United States, he and the other ruling generals here placed a traditional nationalistic interest--taking over the Falklands--above this broad strategic policy.

Interviews with a variety of informed sources suggest that from the onset of the confrontation between Britain and Argentina in late March, Argentine and U.S. officials misjudged each other's intentions and strategic priorities.

Both the Galtieri and Reagan administrations found their leverage evaporated as the Falkland crisis came closer to war. Ultimately, each government repudiated what had become an increasingly controversial relationship for both. The United States moved to support Britain, and Argentina returned to nationalism.

The result has been an abrupt halt in the Reagan administration's efforts to rebuild ties with a country that has a long history of quickly shifting loyalties and even hostility toward the United States.

Relations with the United States, Argentine Foreign Minister Nicanor Costa Mendez said this week, are "bad," adding, "The United States has become allies with the enemy."

But despite a rising tide of anti-U.S. feelings since the Reagan administration shifted to open support for Britain in the South Atlantic, it is not clear here that U.S. ties with Argentina after the conflict will be much weaker than they have been traditionally.

While heavily criticizing the United States, government and political leaders here are careful to stress that Argentina will be open to renewing good relations after the conflict--provided, as Costa Mendez said in a radio interview yesterday, the United States takes the initiative and shows "good will."

The main effect is likely to be modification of Argentina's recently cooled ties with the nonaligned nations and its backing of the United States throughout Latin America in what Galtieri has called the "Third World War" against communism.

"The United States is going to have to forget about Argentina or other South American countries helping them in Central America or other places," said one former government official who, like many others, had opposed the intended shift away from nonaligned neutralism. "They are going to have to solve their problems on their own."

According to a range of sources, U.S. officials, at least at the embassy level, learned even before Galtieri took office in late December that Argentina planned a year of mounting pressure over the Falklands as the 150th anniversary of the British occupation approached. Argentine officials also suggested to diplomats here that if diplomatic efforts failed, Argentina could resort to military action toward the end of this year.

Argentine officials at times attempted to persuade U.S. representatives that Argentine sovereignty over the Falklands could be a key element in their alliance. The British, they pointed out, had done nothing to develop the Falklands, while Argentina could turn them into a base that would protect potentially important sea lanes in the South Atlantic. An official said that when Assistant Secretary of State Thomas O. Enders visited Buenos Aires in early March, Argentine officials requested that the United States "take an interest" in the Falklands dispute. Negotiations between Argentina and Britain in New York were at the point of breakdown, and the Argentines wanted the United States to use its influence to win concessions from Britain for Argentina.

But U.S. officials apparently were not eager to become involved in a 149-year-old quarrel between two allies, and when an March incident on South Georgia Island--the raising of the Argentine flag by some scrap dealers--gradually developed into the beginnings of a crisis, U.S. officials seemingly failed to realize the seriousness of Argentina's intentions.

According to informed Argentine sources here, a small working group in the Argentine Foreign Ministry, had been informed by March 28 that the junta had decided to launch the long-planned military operation.

But U.S. officials, distracted by the immediate dispute on South Georgia, which the United States had offered to mediate, did not indicate they knew the Argentines were seriously planning an armed invasion of the main Falkland Islands until three days later--48 hours before the invasion, according to these sources.

Until then, U.S. officials had been trying to obtain a response from Argentine officials on the placement of Argentine workers on the virtually uninhabited Georgias, but the question of U.S. mediation was left up in the air by Argentina.

Even in the days before the invasion, U.S. officials still thought military action might be held off, and a strong warning was delivered to Argentine government officials saying that any Argentine armed action would damage relations.

Argentine sources, however, tell the story of Galtieri refusing to answer the first two phone calls from the White House on the eve of the attack--while there was still time to call back the troops--to show that Argentina had decided to move regardless of the U.S. response.

Nevertheless, Argentine officials indicate they believed neither the United States nor Britain would react strongly to the seizure of the islands and that it was, in part, the strong new ties with the United States that made the military leadership believe the move was now possible. The United States, they believed, would deplore the move, but would halt any military response by Britain, both out of friendship for Argentina and in the interests of NATO defenses.

It was only when Galtieri talked to Reagan the night of April 1 that the Argentines began to realize that both the U.S. and the British reaction might be far stronger than they had anticipated, Argentine sources say.

But by then, the military leadership had already established its priority, and the taking of the Falklands had been set in front of the U.S. alliance. In the coming weeks of diplomatic efforts by U.S. Secretary of State Alexander M. Haig Jr., no amount of U.S. pressure was able to reverse that decision.

Sources familiar with both sides of the U.S.-Argentine talks last month now say that despite the emphasis by both sides on what Haig called a "hemispheric partnership," relations between the two countries were strained almost from the beginning of Haig's first visit April 10.

The underlying demand of the Argentine government was--as it continues to be--an assurance of Argentine sovereignty over the captured territory. Most of the dozens of hours of talks in Buenos Aires were devoted to formulating specific language that would somehow satisfy this condition.

But a text was never agreed upon by both sides, and in this respect the negotiations never made real progress, according to a variety of sources. At various times, U.S. officials apparently believed they had obtained Argentine agreement to specific wording that might lead to a resolution. But the Argentines would later repeat that sovereignty was not negotiable in their view, regardless of what a carefully worded text might imply.

As the talks dragged on, some U.S. officials seemed to grow frustrated with what appeared to be the lack of decision-making authority among the array of Argentine military and diplomatic leaders. Haig found himself meeting with Costa Mendez, then Galtieri, then the three-man junta or with high officials of the Army, without being able to obtain what seemed to be definitive responses.

Many in the Argentine military, meanwhile, viewed Haig from the beginning as little more than a representative of the British who, as one political source noted, "simply came here to give the same arguments that we have been hearing from the British for 17 years."

Both sides attempted to use the leverage of a break or reversal of their alliance. While at first stressing their respect for the United States, Argentine officials reportedly played up the possibility of a sudden Argentine alliance with Cuba or the Soviet Union.

And in the final marathon weekend of talks in Buenos Aires last month, Haig repeatedly warned that the United States would have to side with Britain if progress was not soon made.

These warnings are said to have provoked little response from the military leadership, however, which replied that it hoped the United States would remain neutral but that Argentina could not yield its claim to the islands.

This seeming intransigence evidently has continued to irritate U.S. officials. But for the Argentines, the choice was finally easy.

"The Malvinas Falklands is a national cause from which it is not possible to retreat," said one political activist here. "The United States has traditionally had the best relations with Argentina. The Haig mission failed because the United States did not want to accept Argentine interests. Now, Argentina could have a good relationship with the United States, but it will never be an ally."