When the Costa Rican government asked the United States for emergency military assistance earlier this month, few people here thought it would be denied.
Some US military officials long have pressed Costa Rica to rebuild its national security forces and to play a larger role in a US strategy of isolating Nicaragua, say numerous prominent Costa Rican politicians.
The requested weapons would be used to beef up security along the Costa Rica-Nicaragua border, where the presence of US-backed Nicaraguan counterrevolutionaries has sparked clashes between the Sandinistas and the Costa Ricans.
Although the $7.85 million requested is a paltry sum compared with US military aid for other Central American countries (the Reagan administration requested $78.5 million for Honduras this year and $243.5 million for El Salvador), it is a huge increase for a country that from 1968 to 1981 received only $5 million in US military aid.
Some former Costa Rican officials think their nation will soon accept even larger levels of aid. They worry that such aid may compromise Costa Rica's traditional role of neutrality and lead to greater involvement in the Central American conflict.
''There are certain sectors of the North American government that want Costa Rica to militarize,'' says a top Costa Rican security official, who asks that his name not be used. ''They want Costa Rica to have a powerful army, military bases, and to participate in regional training. And there are insinuations. But we will keep resisting them.''
Costa Rica has declined US invitations to participate in regional military maneuvers, including an exercise in the Bahia de Salinas off Costa Rica's northwest border with Nicaragua, says Communications Minister Armando Vargas.
The country, which since 1948 has not had an army, also turned down offers to send its police to train at the Regional Military Training Center in Honduras. And it declined invitations to join in regional defense conferences such as the reconvened Central American Defense Council (Condeca), an anticommunist military alliance, Vargas says.
One leader of the party in power, the National Liberation Party, says, ''There's been tremendous pressure'' on Costa Rica to approve a plan to bring up to 1,000 US National Guardsmen to Costa Rica to build roads and airports. If approved by the Costa Rican legislature, it would be the largest contingency of US military personnel on Costa Rican soil in history.
Costa Rica is walking a tightrope, say some National Liberation Party members. It genuinely wants US military aid to rebuild its ragtag security forces. But it does not want to build powerful quasi-military forces and infrastructures that could be used to further US strategic interests in Central America.
The problem is made more delicate by the fact that the US provides nearly $1 million a day in economic aid to the financially troubled nation.
''Because of the world economic depression, Costa Rica so badly needs US economic help, and this weakens our position,'' says Jose Figueres, who has served as Costa Rica's president three times. It was Mr. Figueres who abolished the army in 1948.
By far the most controversial US proposal is the one to bring US National Guardsmen to Costa Rica. The brainchild of US Army Southern Command chief Paul F. Gorman, the plan would theoretically benefit Costa Rica with the guardsmen's construction of roads and airports in remote areas of the country. But the project would also help the US.
''It . . . gives these guys of ours a realistic opportunity to train on terrain that would be quite typical of what they might encounter should they get into a problem in a third-world country in this latitude,'' says the US ambassador to Costa Rica, Curtin Winsor.
The initial plan, introduced last fall, sparked a national controversy here. One of the roads to be built would run north and end a few miles from the Nicaraguan border.
Critics charged this plan was designed to put the squeeze on Nicaragua, that it compromised Costa Rica's neutrality, and that it could lead to a wider US military presence in the country.
''If the guard plan were approved, it would be the beginning of the militarization which we have historically rejected,'' says former Costa Rican President Daniel Oduber.
Under pressure from his party, Costa Rica's current President, Luis Alberto Monge, said no to the plan in January.
In March the US offered a new proposal for public works projects - this time in both the northern and southern zones of the country. The revised plan would bring some 350 to 1,000 US guardsmen to Costa Rica, Mr. Winsor says.
''They (the US guardsmen) would set up what they call a base camp, which is what they would do under realistic conditions,'' the ambassador says. ''They try to make it as realistic a training exercise as possible and to pick projects which would duplicate in reality what they might be required to do in the event of a combat situation.''
The element of this new plan that disturbs some Costa Ricans is the proposed expansion of the airport in Liberia, a city some 25 miles from the Nicaraguan border. The press here has reported that the US is pushing for expansion of a northern airport to receive military cargo planes.
''I have my reservations about Llano Grande (the airport),'' says a key security official. ''It is the most strategic airport in the northern zone and could easily be converted into a military airport.''
US Sen. Jim Sasser (D) of Tennessee, ranking Democrat on the Senate Appropriations Subcommittee on Military Construction, also is concerned about US military plans for Costa Rica. Earlier this month, Sasser asked Defense Secretary Caspar Weinberger to make public the US military plans for Costa Rica. He wonders if the US is seeking an airfield access agreement with Costa Rica and what further arrangements it would seek.
To date, US military aid to Costa Rica has mainly been communication equipment, uniforms, and field gear, because the US cannot legally provide military equipment to foreign police forces.
But House Armed Services Committee member Charles Bennett (D) of Florida is trying to exempt Costa Rica from this restriction. And if the $7.85 million is approved, Costa Rica reportedly will receive light defensive weapons - including jeeps, light antitank weapons, machine guns, and other arms, say US officials here.