The Sandinista victory in Nicaragua is being viewed with both joy and apprehension throughout the rest of Latin America, where the military defeat of President Anastasio Somoza is seen as a stunning setback for U.S. power and prestige in the hemisphere.

No political development since Fidel Castro's triumph in 1959 has so captured imaginations in the countries south of the United States as the guerrillas' triumphant entry into Managua.

Conviccion, a newspaper in Buenos Aires that reflects the rightist views of the Argentine Navy, called the outcome of the civil war "Brzezinski's inglorious Vietnam."

The clandestine lefist Guerrilla Army of the Poor in Guatemala said, "The triumph of the Nicaraguan revolution has caused very important changes in the Central American situation...favorable for the development of our own wars of popular revolution."

The Andean pact countries of Venezuela, Colombia, Ecuadoe, Peru and Bolivia, which each have demorcratic forms of government, issued a communique reflecting traditional Latin American hopes for a liberal democratic government.

"Somoza and his regime personified the most reprehensible practices of oppression and abuse in the history of Latin America," the statement said. "His expulsion from power heralds the beginning of an altogether new stage in the political life of the region."

Few commentators, in the wake of the Sandinista victory, found any reason to praise the United States. Indeed, the right and left seem in agreement that Somoza's fall was a major defeat for U.S. power, leadership and interests in the hemisphere.

Although the Carter administration was actively involved in pressuring Somoza to leave his war-torn country, most Latin American view the United States as having bowed to reality only when it became apparent that Somoza would lose militarily.

Latin Americans form their judgments aware that for more than 40 years U.S. governments strongly supported the rule of the Somoza family.

They laud the Sandinista defeat of what they see as a symbol of "Yankee imperialism" in Central America. They describe the victory as having been achieved with arms and money from Cuba, the Palestine Liberation Organization. Panama, Venezuela and possibly other countries.

It is in this context that both conservatives and radicals were assessing the U.S. failure to change the composition of the five-member provisional junta, backed by the leftist revolutionaries.

The Guatemalan Guerrilla Army of the Poor said in its statement that the Sandinista triumph represents the first victory for leftist revolutionaries in Latin America since Castro came to power 20 years ago. Guerrilla campaigns since then have been frustrated in almost every country in the hemisphere - often with direct U.S. assistance.

The possibility of the rebel movement spreading has aroused the fears of the region's authoritarian governments, such as Guatemala, Argentina, Uruguay, Chile and Paraguay. Some of that concern was expressed Saturday by La Opinion, a newspaper published in Buenos Aires by the Argentine Army. La Opinion printed a map of Central America and the Caribbean showing "the explosive zone where the Nicaraguan case could be repeated."

Guillermo Martinez Marquez, writing in El Mercurio, Chile's most influential conservative newspaper, said that after becoming involved in Africa and the Caribbean, Cuba "has returned to its interventionist policies" in Latin America.

"The United States will soon be surrounded by visible and active enemies with the exception of Canada," Martinez said. "The United States has become geographically isolated.This is where we are, and this is where we will continue, as long as Washington does not decide differently."

What especially has confused the left and frightened the right is that, by not intervening militarily in Nicaragua to install a moderate government, the United States appears to have lost its will to protect what both sides view as U.S. interests.

An Argentine diplomat said his government and others in the hemisphere are attempting to analyze what he called "this new factor," which he said could have an important impact on how anticommunist military governments - and leftists as well - interact with the United States. Latin Americans, until now, have viewed the United States as the overwhelming power in the hemisphere with the will and resources to intervene when necessary to protect its interests.

What the Andean countries, led by Venezuela, clearly are hoping is that Nicaragua, after a period of rebuilding, will become another liberal democracy like its neighbor, Costa Rica. The Andean countries believe that, because they supported the Sandinistas diplomatically, politically and materially while the United States vacillated, they can help shape events there now.

This view, however, is considered naive by many Latin observers, including the respected English-language Buenos Aires Herald.

"At present, needless to say, world and especially Latin American public opinion is wholeheartedly on the side of the Sandinistas," said an editorial in the Herald last week.

"Should they behave as Marxist-Leninists always have, however, the day may not be very distant when it will be realized that evil as Somoza's one-man dictatorship was, it was less evil than the systematic totalitarianism established in Cuba and which may soon be established in Nicaragua, with the outside world tut-tutting its disapproval but utterly helpless to do anything to influence the course of events." CAPTION: Picture, ANASTASIO SOMOZA