With its celebrated "reoccupation" of the Falkland Islands reduced to a grim final stand by a surrounded and increasingly desperate Army garrison, Argentina has begun to veer into an unpredictable course of nationalism, recrimination and potentially far-reaching political turmoil.

The military government of Gen. Leopoldo Galtieri, facing what is increasingly perceived here as imminent defeat by Britain's South Atlantic task force, is clearly fighting for its own survival. As British forces close in on Argentine troops entrenched around Stanley, the military command has amassed what remains of its weaponry and its Air Force in the hope that one all-out battle finally will halt the offensive.

But the junta's real struggle is already turning to the domestic front. In Buenos Aires, the people are being told that Argentina already has "won" its war, regardless of what may happen in the coming days. In the presidential palace, Galtieri has been meeting with civilian, business and labor leaders, promising drastic reversals of economic and social policy to gain support.

On television, military spokesmen are seeking to drown out the sounds of a collapsing national dream with vivid accounts of spectacular--and possibly phantasmic--Argentine victories: an aircraft carrier in flames, a battalion's worth of Royal Marines killed, entire squadrons of aircraft destroyed.

It is no longer clear whether these reports are believed, and many people, in any case, think they will not be enough. "A defeat with honor is still a defeat," said one political leader. "And it is going to be very difficult for the junta to survive that, no matter how it ends."

The sense of crisis is all the more penetrating for political leaders here because it has gathered so quickly. Little more than two weeks ago, with the British fleet seemingly stalled around the islands and negotiations under way at the United Nations, Buenos Aires exuded optimism.

The ruling junta, deeply proud of its record in Argentina's first modern war, seemed convinced that the momentum in the conflict had finally swung its way. There were reports of concessions by Britain, and pressure for peace by European countries. The United States still seemed likely to help Argentina by preventing a British invasion. Weather in the South Atlantic was worsening.

Leaders of political parties, who had begun to attack the government and hint of disaster when the British fleet first arrived, were once again unified with the military. Sensing a diplomatic settlement that would preserve Argentina's place on the Falklands, the parties began elaborate planning for the "postwar era." Elaborate plans were drawn up for transition governments, coalition governments, and even immediate democratic governments. Within the parties, intense maneuvering began for the expected leadership positions.

Now, it suddenly seems that the postwar era will be less orderly than envisioned, that the war, in fact, may go on indefinitely. "A few politicans and government leaders thought they were going to be able to structure and control the changes in the country after the war," said Mario Campora, a former leader of the Peronist political party. "They had this dream of a kind of belle epoque.

"But we are beginning to see now that the forces created by this crisis are going to be undirected. No one knows what is going to happen, only that the country is going to go through some profound conflicts."

As Argentina's hopes of hanging onto the islands have unraveled, the military government and the civilian leadership have seemed to loosen their grip on the old political order, abandoning longstanding policies to align with past enemies almost overnight.

Galtieri, whose government was portrayed here as leading Argentina into a new strategic alliance with the United States and the West, has suddenly pronounced himself a Third World leader. The Falklands conflict has become the focal point of the North-South struggle, Galtieri says, and Argentina has become strongly nonaligned.

In meetings with business and labor groups in the past few days, Galtieri has also promised to change the conservative, free market-oriented economic policy the military has followed for the last six years.

Now, only five months after his inauguration, Galtieri has come close to endorsing the traditional program of the military's oldest enemies, the nationalist, populist Peronists, named for the three-time former president, Juan D. Peron. Analysts close to the military here say that Galtieri, by radically changing his administration's politics to those of fervent nationalism, could maintain the military in power long beyond the present conflict.

But the military leadership is far from agreed.

Even as Galtieri and Air Force commander Basilio Lami Dozo have promised economic policy changes, Economy Minister Roberto Alemann has said in repeated interviews that no such change has been agreed. And though Galtieri and his interior minister have insisted that the military will stick to its own plan for returning Argentina to democracy gradually, Lami Dozo said last night that he might support a new, "emergency" government composed of military and civilian figures.

Galtieri also faces strong opposition within the Army to some of his recent policy shifts, according to sources close to the military command. At a recent meeting with the 10 division generals who form a top rank of authority in the Army, Galtieri heard strong criticism of his decision to withdraw Argentine representatives from the Inter-American Defense Board in Washington and the the U.S. military school in Panama, these sources said.

Several ranking generals told Galtieri that Argentina could not freeze its relations with the United States, but instead had to look to Washington for a solution to the crisis. Galtieri, according to sources, hotly responded that he would no longer cooperate with the Reagan administration, but was backed by only one of the generals present.

The civilian leadership itself is near turmoil. The labor movement, on the point of uniting before the Falklands invasion, has now redivided into two increasingly hostile camps, one of which has promised to mobilize its followers for "progressive change." The Radical Party, which represents middle-class interests here, is embroiled in an intense leadership struggle. The multiparty front of the five largest civilian parties has not yet issued a statement on the Falklands crisis.

The Peronists, the largest political party, are mobilizing on their own. A statement last night called for the organization of "defense committees for national sovereignty."

No one in the parties seems to know what will happen if the islands are lost, or what they will attempt to do. But most are worried that Argentina will enter a time of intense factional struggle.

It seems clear only that with or without the junta, the "war with Britain" will not be renounced. The country will lead Latin America and developing nations in a struggle against the "Anglo-Saxon superpowers." If it has to, it will make a pact with Cuba, or the Soviet Union, political leaders seem to agree.

During the past 10 days, toleration of moderate spokesmen seems to have disappeared. Now, to question Argentina's militancy in the long crisis is to be accused of treason. To oppose a radical shift in the country's policies is to be identified with anti-Argentine plots or--worse--the U.S. Embassy.

The military command will not be blamed for invading the Falklands, many analysts here believe, and it may not even be faulted for leaving the islands to the British task force. Argentina, political leaders say, has conducted itself well, inflicting damage on British ships and fighting with a determination few outsiders expected.

Instead, Argentina's political leadership has found a much easier culprit for the loss of the islands: the United States. It has only been the U.S. material support for Britain that turned the tide of battle, they say.

Within the military command itself, however, responsibility for the loss will not be so easily shed from the high command, it appears. Already, the infighting has begun among military commanders, sources here say, with each service and each high officer seeking to escape the blame for what appears to be happening.

In the Army, it is said that Galtieri will quickly relieve the commander of the corps entrusted with the Falklands operation, along with the island's military governor, Gen. Mario Benjamin Menendez. For the Navy, the recriminations are beginning over the Argentine fleet, sources say, which has apparently clung to the Patagonian coast.