Buenos Aires — After an opening round of war talk, and a period of relief with US mediation, Argentina is grimly bracing for war with Britain.
The military government has placed a considerable part of its forces on alert and is preparing the Argentine public for a major confrontation.
The Argentine military command has been steadily reinforcing its position on the conquered Falkland Islands with new troops and supplies carried in by air.
The number of Argentine troops and other servicemen now based on the islands is reported to have reached 8,000 to 9,000. A unit of Mirage jet fighters has also been stationed on the islands.
According to a foreign specialist in such matters, the Argentine military buildup is acquiring a ''momentum of its own'' which might now be extremely difficult to reverse.
''They are just pouring the stuff into the islands,'' this specialist said.
While the military government has found little support for its takeover of the islands in the outside world, it has clearly gathered massive support within Argentina. Nationalistic emotion is running high here.
Thousands of civilian volunteers, including a few octogenarian veterans of past military service, have been lining up to help with the defense effort. They have been offering their services for civil defense and medical aid, among other things. Indeed, the government has had many more volunteers than it can use at this stage.
In Comodoro Rivadavia, the southern port that serves as an advance command post and base for an air bridge to the islands, Argentine military authorities have announced that the region is ''saturated'' with volunteers. They have urged further volunteers not to come unless they are called, because the government lacks the means to feed and lodge them.
Towns along the southern coast have been practicing air raids and blackouts. Evacuation plans have been distributed. Hospitals have been prepared to accept casualties should war come about.
At Puerto Belterano, where much of the Argentine fleet is based, naval officers have declared that the Navy is now in a state of alert. Adm. Isaac Anaya, Navy chief, said ships are ready to move, ''at a moment's notice.''
Argentina's economy minister, Roberto Alemann, announced that the government is establishing a ''patriotic fund'' in order to receive donations from Argentine businessmen aimed at helping finance the military occupation of the Falklands. According to a banker here, a considerable number of Argentines began withdrawing their bank deposits last week out of fear that a ''a patriotic tax'' would be imposed on them.
There is some concern here, meanwhile, that a war between Britain and Argentina could expand beyond the borders of Argentina. To start with, Bolivia has offered the support of its Air Force to Argentina. In La Paz on April 12, the commander of the Bolivian Air Force, Gen. Natalio Morales Mosquera, announced that Bolivia was moving a group of fighter planes close to the border with Argentina.
Peru and Equador have also hinted that they might be willing to provide military support to Argentina.
Some sources fear that if Argentina is successful in its conflict against Britain, it will next try to resolve by military force its dispute with neighboring Chile over the tiny islands of Lennox, Picton, and Nueva in the Beagle Channel.
The result of all this could be a destabilization of the entire region.
But according to the Argentine newspaper Cronica, Argentina's President Leopoldo Fortunato Galtieri told US Secretary of State Alexander M. Haig Jr. during Haig's visit here this past weekend that if Britain began to put military pressure on Argentina, this country would accept assistance from wherever it could get it, including the Soviet Union.
The Soviet Union, already the leading importer of Argentine grain, recently signed important trade pacts with Argentina and agreed to supply this country with enriched uranium for its nuclear development.
Argentina is believed to be capable of building an atomic bomb within the next few years. Some sources think the conflict with Britain will encourage Argentina to accelerate its nuclear program.