Nestled at the edge of San Jose's bustling international airport, two hangars house a half dozen small propeller planes and a two-seater bubble helicopter. On the grass outside, partially overgrown by weeds and clearly long disabled, a camouflage-painted Sikorsky chopper lies on its belly.

"Welcome to the headquarters of the air force of Costa Rica," Information Minister Armando Vargas says solemnly, just before breaking into a laugh.

Government officials here seem to take pride in exposing their country's military weakness, from its handful of unarmed aircraft and four-boat navy to the poorly equipped 6,000-man police force that is constitutionally barred from turning itself into an army.

But after 35 years of relatively peaceful existence as a regional anomaly, with a stable democracy and no declared internal or external enemies, Costa Rica is finding it increasingly difficult to remain aloof from the bloody conflicts sweeping the rest of the isthmus.

A longtime avid and often amused observer of political strife in neighboring Nicaragua, Costa Rica now sees the problems immediately across its northern border and beyond as a threat to its own political system, its fragile economy and the image of international neutrality it has cultivated carefully for decades.

Nicaragua's Sandinista government has charged that escalating attacks along its southern frontier led by exiled guerrilla hero Eden Pastora have been launched from inside Costa Rican territory. While officials here issue public denials, they acknowledge privately that they have little control over rebel activity and note the difficulty of stopping a movement that has widespread public support in Costa Rica.

In the capital, rival representatives from countless factions in the various Central American wars gather in freedom to plot and plan and hold press conferences, leading to charges that Costa Rica condones their actions.

Despite its gratitude and continuing desperate need for U.S. economic aid, the government visibly winces each time the Reagan administration loudly proclaims that Costa Rica shares its concern and supports its policies in the region.

As a result, officials here said, longtime democratic friends in Western Europe and Latin America, whose help also is needed, have begun snubbing San Jose as they see the long arm of Washington draped around Costa Rica's shoulder.

"We feel a little bit alone," President Luis Alberto Monge said in an interview .

In what he described as an act of near desperation and critics inevitably called a response to U.S. pressure to involve other nations in its anti-Sandinista campaign, Monge has called for a multinational Latin American force to oversee Costa Rica's border with Nicaragua and certify its neutrality. The effort has been met with suspicion and a thinly veiled reluctance to help.

"We are not prepared for a situation of this magnitude," Information Minister Vargas said, "but we don't want to be, either. We don't want to fall into the trap of creating another power base here" by reestablishing the army disbanded by law a quarter century ago.

"The Nicaraguan problem is a problem inside Nicaragua, of Nicaraguans fighting Nicaraguans," Vargas said. "At this time of economic crisis for us, Costa Rica does not want to be involved."

Yet, whether its government likes it or not, Costa Rica is involved in Nicaragua's problems, for reasons of geography, of history and the clear desire of many of its own people to play a role.

One of the first challenges to the newly pacifist Costa Rica during the early 1950s was an attempt by a disgruntled would-be president to overthrow the government here with the help of Nicaraguan dictator Anastasio Somoza Garcia, the father of the man the Sandinistas ultimately ousted.

By the time the Sandinistas were ready to make their move against the son, beginning in 1978, Costa Ricans were only too happy to lend their territory and their support to the effort.

"We were the ones who gave them guns," said Public Security Minister Angel Eduardo Solano. "It was here that the planes landed with arms. They would have been nowhere without us."

But if there is one thing Costa Ricans dislike as much as they did the Somozas, it is communism. Once the Sandinistas veered to the left with Cuban help, Solano said, Costa Ricans felt "betrayed."

The troubles inside Nicaragua coincided with Costa Rica's worsening economy and a series of strikes and political pressures from the domestic left that many here believed were at least encouraged by Cuba and the Sandinistas themselves. When Monge assumed office one year ago, Costa Rica was officially in a state of crisis. As Washington quickly came to his aid, Monge soon became one of the Reagan administration's most visible friends among Latin American democracies. While not exactly echoing the adminstration position on Nicaragua, Costa Rica rarely disagreed.

"Of course you're going to be nice to someone who's going to give you millions of dollars," former president Daniel Oduber said. Yet he and Monge agreed that no Costa Rican president could survive domestically on an anti-American platform. "Costa Rica is basically a pro-Yankee country," Oduber said, "perhaps the only one truly so in all of Latin America."

It is only recently, however, that the cost of such closeness has begun to approach the benefits.

Monge said he does not share Washington's belief that the mere existence of a Marxist government in Nicaragua poses a threat to regional stability. If the Sandinistas would behave, he said, Costa Rica would be "prepared for peaceful coexistence with a neighbor that has a different philosophy."

Costa Rican officials insist that despite an antipathy toward the Sandinistas and a fear of leftist expansionism, they truly want to remain as uninvolved as possible with U.S. security strategy in the region.

"The responsibilities and interest of the United States are different from those of Costa Rica," Monge said. He has offered San Jose as a site for U.S.-opposed open dialogue between El Salvador's beleaguered government and its guerrilla opponents.

At the same time, at the end of his first year in office, Monge has succeeded to a large degree in stabilizing the Costa Rican economy. The strikes and subversion here have largely ceased, due in the eyes of many officials to Costa Rica's pledge to stay out of Central American battles and spend its money at home.

But as the border situation has deteriorated over the past several weeks, Costa Rica says it fears that the gains of the past year could be lost and that a public display of unarmed weakness will no longer provide enough defense.