Argentina's military government, battered by an apparently unforeseen and increasingly costly war with Britain, is now seeking to change what was originally portrayed here as a blunt assertion of territorial claims into a Latin American struggle against the traditional power structure of the West.

At the same time, the administration of President Leopoldo Galtieri has revised its rightward-leaning foreign policy to embrace once frowned-upon alliances with the nonaligned nations group and liberal Latin American governments.

Argentina is no longer simply fighting over the control of a small group of islands, Foreign Minister Nicanor Costa Mendez told a press conference last week. Instead, he said, it is striking a blow against "the international status quo" since World War II that awards control of the West to the United States and Western Europe. By fighting back, Costa Mendez and a variety of high Argentine officials have maintained recently, Argentina has come to embody the effort of developing nations, especially in Latin America, to assert themselves both politically and economically against the superpowers.

Costa Mendez said in the press conference that he believes Argentina's battle would have a "profound effect on the development of international relations and would considerably improve understanding between North and South."

This improvised ideology has been made increasingly plausible, Argentine analysts here note, by expanding British military action in the South Atlantic, which has offended once neutral Latin American countries while overshadowing Argentina's initial, nearly bloodless invasion.

The shift by the United States from neutrality to open support for Britain also has helped the Argentine political cause, by increasing the appearance here of a superpower enforcement of an unjust order.

Now, after initial international isolation, Argentina is finding at least diplomatic support in increasingly strong terms for its arguments among South American nations that once looked with distaste on the military junta and its perceived belligerence.

Brazil, which initially took a neutral stance on the conflict, has strongly condemned in the United Nations Britain's latest attacks on the islands and reportedly has sold surveillance airplanes to Argentina. In addition, Venezuelan President Luis Herrera Campins has called British attempts to retake the Falklands "an aggression against Latin America."

Internally, spokesmen who in recent weeks began to question the government's judgment in the crisis quickly seized the military's new justification for fighting Britain.

Several Argentine civilian figures have argued, as the moderate newspaper Clarin did in a long analysis yesterday, that the conflict had destroyed "the myth" that "the destiny of Argentina consists in taking a complementary role to other powers in the West."

For the country's traditionally nationalistic political leaders, the Falklands occupation has come to represent what they believe will be a long overdue recognition by Western powers of Argentina's political and strategic importance. As a result, there has been little complaint so far over the military's determination to fight Britain's powerful military forces, despite heavy Argentine losses.

The broad ramifications now attached to the fighting by the government seem to have little to do with Argentina's initial motives for the April 2 invasion. According to widespread reports here, the small military group that planned the operation acted impulsively and did not believe that their action would result in a war or even a significant international confrontation.

As a result, Argentina explained the invasion initially in strictly parochial political terms. In a national television address the day of the invasion, Galtieri made no mention of North-South relations or Old World colonialism--the issues that have now become the centerpiece of Argentina's stand.

Instead, he explained simply, the decision to invade was taken because of the "lack of recognition of Argentine rights," over the territory, which Argentina has claimed from Britain since 1833.

Saying the military forces had acted to "save national honor," Galtieri said "the decision resulted from a necessity to put an end to the interminable succession of evasions and delays" by Great Britain in turning over "territory that by legitimate right is part of the national fatherland."

This explanation was consistent with decades of Argentine foreign policy, which had pictured Argentine possession of the Falklands not only as a right but as a key to its destiny to control a large part of Antarctica.

The key to Argentina's more elaborate present position is that the Falklands are "a vestige of colonialism" maintained by an extracontinental power, an argument Argentina frequently made in international forums in past years in seeking support for its historical claim to the Falklands. In itself, the issue of territorial claims on the islands has long been clouded by a series of 19th century settlements, expulsions and treaties involving a variety of nations.

But the colonialism argument, while attractive to Latin American countries, can become difficult at times in Argentina's case, as CostaMendez demonstrated in his press conference last week.

After discussing at length the colonial nature of British rule on the islands and U.N. resolutions calling for the end of colonial governments, Costa Mendez argued that Britain had been obliged by the United Nations to end the "anachronistic colonial situation" on the Falklands.

Costa Mendez then went on to argue that the islands' residents should not be granted the right of self-determination--as specified by the same U.N. provisions--because "they are not a colonized people."

Thus, in seeking international support against the British, government spokesmen repeatedly have described the British efforts to retake the Falklands as an act of colonialism, rather than an attack on Argentine territory, and the British task force insistently is labeled as the "colonialistic fleet." Simultaneously, Argentina has opposed strongly any attempt to give the island residents a measure of power in the future administration of the islands, arguing that the Falklands, at least in this sense, are not a colony.

In recent weeks, however, this intricate colonialism argument has been expanded into an issue of the right of developing countries to act in their own interests without the approval of Western superpowers.

In a major speech last week, the Navy commander in chief, Jorge Anaya, portrayed the conflict as the beginning of a new relationship in the West, with Latin America taking the lead from decadent Northern powers.

"We're the protagonists of a historic episode that we have neither sought nor provoked," Anaya said. "However, it has allowed us to understand, both Argentines and all of humanity, the reality of the world we live in, which no longer admits the superpower."

"When the world breaks up, timid to face our true philosophy of life, Latin America rises up capable of converting itself into the renovating savior of a West exhausted in its convictions," Anaya added.

Argentine military leaders have taken similar positions in the past in explaining why the United States and Western European countries have criticized them for their violent campaign against internal opponents during the 1970s, which the military regarded as a key battle in what they call the "Third World War" against communism.

In this case, however, the Galtieri government has backed up its rhetoric by suddenly reversing its intention to cut its ties with the nonaligned nation group in favor of a stronger alliance with the United States.

In recent days, Galtieri and Argentine Foreign Ministry officials have taken care to identify Argentina with the nonaligned countries, and there have been reports here that Costa Mendez will lead the next Argentine delegation to the next nonaligned nations meeting in Cuba, once identified as one of Argentina's worst enemies.

Argentina also has sought to heal quickly longstanding differences with Latin American rivals. This week, the government suddenly agreed to allow the departure of a former Argentine labor leader who had been living in the Mexican Embassy for the last six years and whose case had become a source of rancor between the Argentine military and Mexico.

The extent to which Mexico and other Latin American countries that have long distrusted Argentina will embrace the Falkland Islands conflict as their own will be seen if Argentina attempts to win sanctions against Britain by these countries in the Organization of American States. So far, Argentine officials have held off from such a move, saying they did not have sufficient support.

Internally, however, Argentina's philosophy of its struggle already seems to have a significant effect. Argentina's success in forcing Britain and other Western countries to recognize it as a formidable military adversary, rather than a rickety Latin American power, has become a major theme of political commentary in Buenos Aires. Many Argentines now seem to believe that Argentina's battle with Britain will turn it overnight into a power to be reckoned with in the world, an idea consistent with the country's nationalistic view of itself.

"Argentines are extraordinarily ambitious for their country," wrote James Neilson, the editor of the Buenos Aires Herald, before the Falklands crisis began. "Despite everything, they assume that Argentina is already great and that one day this will be accepted by everyone . . . . Greatness is not something Argentines must achieve, it would seem, but something that the world one day will have to recognize."