Buenos Aires — Argentina was engulfed at midweek in a mood of unsettling apprehension and pessimism.
Amid growing indications that British commandos had already gone ashore in the Falkland Islands, the military government suddenly had second thoughts about its earlier rejection of United States mediation.
At the same time, speculation began to surface here about the possibility of the junta collapsing - perhaps to be replaced by an extremist government of the right or the left. Such talk was perhaps premature. But the fact that it was being heard at all suggested the jitters being felt here as the war clouds formed.
Argentines still find it hard to believe that they may soon be engaged in their first war in 120 years. But this sort of disbelief is being steadily replaced by the realization of the gravity of the situation in which Argentina finds itself. (Unconfirmed rumors were circulating in Argentina at time of writing Wednesday that the British destroyer Exeter had been attacked by Argentine special forces still at large on South Georgia.)
The strain clearly is being felt by the embattled government of Gen. Leopoldo Fortunato Galtieri. Lights in the Casa Rosada, the Argentine White House, burned into the wee hours of the morning Wednesday as General Galtieri met with fellow Army, Navy, and Air Force officials. And later during the day the junta reversed its rejection of US mediation and announced it was looking at new proposals by the Reagan administration.
The American proposals include both an Argentine and British pullback of forces from the Falklands and the positioning of a US troop presence on the islands. It was even suggested here that Secretary of State Alexander M. Haig Jr. might return at week's end for talks with Argentine officials.
Meanwhile, the military government, clearly on the defensive, began issuing a variety of directives and communiques - banning strategic exports, restating determination to hold onto the Islas Malvinas (as the Falklands are called here) , asking Argentines not to hoard food or medicines, and telling them not to withdraw money from banks. The government also warned against putting much stock in ''false versions'' of victory emanating from London.
Such efforts, however, could not stop the anxieties felt here in the wake of the British recapture of the South Georgia islands (still not admitted here) and the imminence of the expected British move to retake the Falklands.
The government officially took heart that the foreign ministers of the Western Hemisphere had voted early Wednesday to recognize ''Argentina's sovereignty over the Malvinas Islands.'' But unofficially there was not only disappointment but anger that the foreign ministers had not gone further in support of Argentina.
The vote clearly added to Argentina's sense of growing isolation as it struggles to meet the British challenge in the South Atlantic. The Galtieri government had hoped for more than emotional and rhetorical help from Latin America. It did not get it.
Moreover, the abstention of four nations - the US, Colombia, Chile, and Trinidad-Tobago - clearly shocked the government, particularly in view of the watered down nature of the resolution.
There was particular concern here over Chile's abstention as reports reached Buenos Aires that Chile has offered the British Navy use of its important southern port of Punta Arenas in Tierra del Fuego at the bottom of South America.
''But the military has to realize that it got itself, and the Argentine nation, into this mess,'' comments a longtime Argentine political observer, ''and really only has itself to blame - even though the initial seizure of the islands was supported by most Argentines, myself included.''
More and more, the government is being criticized by Sunday night quarterbacks here for having launched the Falklands takeover April 2 without much thought of the consequences. At the time, the Argentine economy was recognized to be in poor shape - and it has grown steadily worse in the days since.
Just this week, three auto manufacturers - Fiat, Ford, and Renault - shut down all production lines because automobiles have not been moving off showroom floors.
That is regarded as only the tip of the dangerous economic iceberg. Newspapers are full of accounts of business bankruptcies, soaring interest rates , problems caused by the shutoff of British and West European markets, and high inflation and unemployment levels.
As if this was not enough, the government was reportedly ready to issue already printed 10 million peso bills. Already the 1 million peso bill, worth roughly $80, has become commonplace with the 500 peso note (about 4 cents) disappearing from use.
Military preparations, meanwhile, continue. But there has been a noticeable decline in the number of photographs of military units and military equipment in newspapers and on television. Reports coming from Patagonia in southern Argentina, where much of the military is presently concentrated, are much less frequent and much less detailed than earlier.
Foreign newspapermen have been ordered out of such southern cities as Bahia Blanca, which is the big naval base, and other Air Force and Navy bases, including Comodoro Rivadavia, Rio Gallegos, and Trelew. In addition they have been ordered to leave Ushuaia, the southernmost city in the world where Argentina has naval facilities.
On the Falklands, where Argentina has at least 10,000 troops, there are now indications that the majority of these soldiers, most of whom are doing their one year obligatory conscription service, are from northern Argentina where the climate is warm and pleasant. Reports suggest that they are having a difficult time adjusting to the cold and blustery winds already sweeping across the South Atlantic and the Falklands in this early auturmn.
The weather is going to be a factor in any British efforts to retake the Falklands. The longer the mighty British armada is at sea, the more difficult becomes the task of maintaining and provisioning the fleet. Argentine military sources cite this as one reason they expect an early British effort to retake the Falklands.
The British in London have announced an air blockade of the Falklands, beginning at 7:00 a.m. (EDT) Friday. Such a blockade, added to the sea blockade already in effect, will make Argentina's reprovisioning of the islands and its troops there increasingly difficult and suggests to people here the British determination to reoccupy the islands - sooner or later.
The mass circulation morning tabloid Clarin Wednesday said flatly in its front-page headline ''Imminent attack on the Malvinas.''
Clarin and almost all other Argentine newspapers - the conservative La Prensa , the more moderate La Nacion, and the afternoon La Razon - sell out almost as soon as they reach the newsstands, so insatiable is the Argentine public's curiosity over what is happening.
It still is difficult for Argentines to believe that their own nation and Britain, a country with so much influence here, are about to go to war, if indeed they are not already at war.