As Britain prepares for a showdown, Argentina vows fight 'to the finish'

As the British steadily tighten their noose around Port Stanley, with its recapture expected momentarily, it becomes apparent that British success in the Falklands campaign rests on two key factors:

* The British have inflicted heavy losses on the Argentine Air Force--a toll not only in aircraft, but also in pilots. This made possible the landing of British troops at San Carlos Bay on East Falkland 10 days ago with a minimum of casualties.

* The San Carlos Bay beachhead gave the British a ground base--a land enclave that was won quickly at heavy Argentine expense. Establishment of this beachhead allowed the British to move some of their ships out of range of mainland-based Argentine planes.

Argentine military sources in Buenos Aires privately admit the British have the edge in the Falklands campaign despite official government announcements to the contrary and the steady drumbeat of optimism expressed by Gen. Mario Benjamin Menendez, commander of the embattled Argentine garrison on the Falklands.

The bulk of that garrison is concentrated in and around Port Stanley, where General Menendez vows to fight ''to the finish.''

But key military figures in Buenos Aires are beginning to admit, in effect, that the game is up. ''The British now appear to have gained a slight superiority,'' is the way one key Army colonel put it this week. ''We may now have to change our strategy.''

What change in strategy could save the day for Argentina on the battlefield is, however, unclear. Only the Argentine Air Force has proved a serious challenge to the British. But its losses have been heavy. The other two Argentine services do not seem to have the capacity to take part in a change of strategy.

The Argentine Navy is virtually counted out of the struggle. It stays close to its mainland bases, far from the fighting. Argentine naval officials privately admit their fleet is largely antiquated. They may be afraid to send it to sea for fear of losing it.

The Argentine Army has put up some stiff resistance to British troops on the Falklands. But although they are outnumbered, Britain's professional soldiers have proved more effective in combat than the largely conscripted Argentine Army.

As a result, the Argentines have suffered heavy losses. British Defense Ministry spokesmen talk of 250 Argentine fatalities in fighting around Goose Green and Port Darwin over the weekend--compared to only 16 British dead.

British forces at Goose Green charge that Argentine forces there raised white flags, then reopened fire. This, they say, resulted in some of the British deaths.

In addition, British forces report having found tanks of napalm at Goose Green and instructions for mounting drop tanks of the substance to Argentina's Pucara aircraft.

For the British on East Falkland, it is now ''on to Port Stanley,'' as a British commander told a British reporter on the scene.

Port Stanley is likely to fall quickly now that the British cordon has been firmed up. With the capture June 1 of the Mt. Kent overlook 10 miles from Port Stanley, Argentina's military position on the island looks bleak.

Behind this steadily worsening situation, moreover, are the withering losses suffered by the Argentine Air Force in the past 10 to 14 days. At least 70, and perhaps as many as 90, Argentine aircraft have been downed or destroyed on the ground in the past month--out of an Air Force that originally numbered between 140 and 220 craft. By Argentine admission, many of them were outdated.

Even more important than the loss of planes, however, has been the loss of experienced pilots, many trained in the United States. It is easier to replace planes, as Argentina appears to have done on a limited scale in recent weeks, than trained pilots.

It is estimated that 25 sophisticated Argentine jets--US-built A-4 Skyhawks, French-built Mirage IIIs, and Israeli-modified Mirages called Daggers--were downed in aerial combat in the past 10 days, almost all with the loss of their pilots.

Britain has also inflicted heavy damage to Argentine planes on the ground--in raids such as the May 15 attack on Pebble Island. That raid got rid of aircraft that could have been used against the British fleet when the San Carlos Bay beachhead was established.

The British may also have struck quietly at the Argentine mainland. A confidential Argentine military report is understood to admit a British commando raid on the Rio Gallegos Air Force base in southern Argentina and the destruction of several French-built Super Etendard fighter bombers. Argentina was thought to have had five Etendards when the war began. An Exocet missile fired from one of these bombers honed in on the British destroyer HMS Sheffield in early May. Set afire, the Sheffield eventually sank.

In retrospect, that incident was probably the high-water mark of the Argentine struggle against the British.

It caused consternation in British naval circles and helps explain the strenuous British effort to knock down as many Argentine planes as possible and to get its Falklands beachhead established.

Once Britain had a strong contingent of troops on the Falklands, along with several island air bases, it was possible to move some of its key ships out of range of the mainland-based Argentine Air Force.

But, according to Argentine military sources, it was the winning of air superiority over the Falklands that enabled the British to get the edge in the battle. Few Argentine aircraft are now being sent out from mainland bases to take on British jets, strike at the ships of the Royal Navy, or attack the ever-widening British beachhead on the Falklands.