On the surface, the resignation of El Salvador's defense minister and his replacement by the head of the country's National Guard might seem little more than another round of musical chairs.
Gen. Jose Guillermo Garcia, after all, now joins a lengthening list of leading Salvadorean government officials who have stepped down, fled into exile, or shifted their allegiance to the guerrillas since the 1979 coup.
And his successor, Gen. Carlos Eugenio Vides Casanova, comes from an institution still under heavy criticism for human-rights violations - especially since five National Guard officers stand accused of involvement in the murder of four American churchwomen on Dec. 2, 1980.
Yet the change itself, announced by General Garcia at a news conference in San Salvador April 18, has broader implications:
* It probably heralds major shifts in Salvadorean military command structure.
* It hints at the extent of United States influence in Salvadorean affairs.
* It suggests that democratic institutions are working - in a part of the world where changes in military and political leadership are often accomplished at the point of a gun.
Western diplomats contacted in San Salvador April 19 see the appointment of General Vides as the first of a number of high- and medium-level changes in the military due to be announced in the next few days - perhaps even including the replacement of Col. Francisco A. Moran, a Garcia protege, as head of the feared Treasury police.
General Garcia, who early on had the support of US officials, came under increasing criticism over the past year as leftist guerrillas gained strength, skill, arms, and penetrated urban areas.
The former defense chief has been blamed for directing a five-day-a-week, 9 -to-5 war - operating large, noisy sweeps through the countryside, rather than (as US military advisers have advocated) organizing his troops into small patrols to fight guerrillas at night.
His replacement, General Vides, has a reputation for being cooperative. And it is thought he may be more willing than Garcia to adopt new battle techniques - although he has little experience on the battlefield or in planning military strategy.
''People generally have confidence in (Vides),'' says a US official. ''For us , he's been one of the most cooperative officers.''
He is known, says a military source, as ''everybody's second choice'' to succeed Garcia, a compromise candidate agreeable to military, political, and US interests.
The question remains, however, about Vides's ability to wage a tougher war against the guerrillas. His experience, which includes a stint at the Command and General Staff School in Peru in 1969 and 1970, has mostly been administrative. During his tenure with the National Guard, he is credited with cashiering a number of officers suspected of being involved in human-rights abuses.
But ''he doesn't really have to be a military strategist,'' says a US official, as long as he is willing to ''reward efficiency and punish inefficiency'' in making his appointments.
General Garcia, who shared the ideals of the reform-minded military junta responsible for the 1979 coup, was the last member of that junta to remain in office. As a proponent of land reform, he won the support of Washington officials, although he was roundly criticized by the Salvadorean political right.
But Garcia's strengths, according to Western observers, lie less in military strategy than in political maneuvering. He is said to have built up a following by promoting field commanders more for their personal commitment to him than for their military merit.
Criticism of Garcia's conduct of the war came to a head in January, when Col. Sigifredo Ochoa Perez, the Army's commander in the northern Cabanas Province, refused to take orders from Garcia and demanded his removal.
His rebellion (which brought into the open a growing undercurrent of criticism of Garcia within the armed forces) ended when he was sent to Washington as a military attache. But he stepped down only after reportedly extracting a promise that Garcia would resign within 90 days. In March, Garcia submitted his letter of resignation to President Alvaro Magana, who took no action on it until threatened last week by a mutiny from Air Force Cmdr. Juan Rafael Bustillo.
Also of note in El Salvador:
As the nation gears up for next December's presidential election, popular former President Jose Napoleon Duarte has won the nomination of the Christian Democratics.
At an party convention April 18, he defeated Foreign Minister Fidel Chavez Mena, who was seen by some as a more acceptable partner if the Christian Democrats had to form a coalition with right-wing parties.