US stand on Salvador: 'calculated ambiguity' . . . or evidence of Washington's indecision?

The focus in world affairs over the past week has continued to be on what the United States might do to Fidel Castro and his projects in the Caribbean because of El Salvador.

According to US Secretary of State Alexander M. Haig Jr. the US ''will do whatever is necessary'' to support and protect the ruling junta in El Salvador from what the Reagan administration believes to be consistent and continuing aid from Cuba to the rebels in El Salvador.

Uncertainty about what might be deemed ''necessary'' was stimulated by other remarks. The assistant secretary of state for inter-American affairs, Thomas O. Enders, told Congress that ''nothing has been ruled out.'' Could that mean that the White House in Washington is thinking of possibly sending US armed forces to Cuba to put a final end to the Castro era there?

White House press secretary Larry Speakes asserted that ''the President has said he has no plans to send troops anywhere -- at the moment.''

When officials are questioned about the possible meaning behind these and similar statements about policy on events in Central America, they are frequently told that the administration is employing the technique of ''calculated ambiguity.'' They want to keep others ''guessing.'' There is no point, they say, in telling Fidel Castro and others ''down there'' what ''we won't do.''

Is it ''calculated ambiguity'' or an unresolved argument inside the administration over what should or could be done?

Early in the Reagan administration a study was made of all possible actions which might be taken against Fidel Castro toward ending his reign in Cuba and his capacity to continue to make trouble for the US and its interests and policies in his neighborhood. Those studies ranged from the military invasion of Cuba to seeking a negotiated accommodation there. Similar studies are understood to be going on again now.

Such studies continue to expose the difficulties involved in taking the kind of strong military action which has been implicit in the rhetoric of the Reagan administration from the days of the 1980 campaign down through the early days in office. The record is full of promises to end the Castro ''nuisance'' ''at the source.''

Offhand, it would seem to be an easy matter. Cuba is an island surrounded by water that is dominated by the US Navy. It lives under skies dominated by the US Air Force. Its population is only about 10 million. Surely, the United States with its enormous military power could stamp out Senor Castro's puny military forces with a little finger.

But when the Pentagon ''staffs out'' the problem of a Cuban invasion, it comes up with disconcerting conclusions. The Cuban Army may be small (about 200, 000 men), but it is tough and has been battle-hardened in Angola, Ethiopia, and other places. There is a substantial militia, probably about 100,000. It is calculated that Army and militia would fight against any invasion. The job could be done, but the cost in terms of casualties could run very high. A US force as big as the one committed to Vietnam might be required to do the job, and with no certainty of quick success.

Complicating the matter is the fact that the Soviet Union gives every indication of caring about what happens to Cuba.

Were there no Soviet Union in the equation, the US could smother Mr. Castro by putting a tight blockade around the island. But Moscow might decide to challenge the blockade. In that case the attempt to smother Castroism in its cradle would lead into an unpleasant and dangerous confrontation with the Soviet Union.

In such a confrontation, how many friends and allies would come forward to join the battle? One friendly diplomat remarked, not cynically but factually, ''You might get Chile and the Argentina.'' It is unlikely that there would be others.

Mexico, the most important other country in the region, opposes action against Cuba or in support of the ruling junta in El Salvador. France supplies Nicaragua with weapons and favors a negotiated settlement in El Salvador. No important ally in Europe views events in the Caribbean through Washington lenses.

Most of the outside world would view a US resort to force against Castro in Cuba or in support of the ruling military-civilian junta in El Salvador as being as reprehensible as the use of military force by the Soviets in Afghanistan or in Eastern Europe. An American military intervention anywhere in the Caribbean would bring down on Washington a vote of censure in the UN. Friends and allies would abstain. But the majority of nations would vote to censure. The NATO alliance would be further strained and weakened.

So after a hard look at the idea of striking at ''the source'' of Caribbean unrest in Cuba, the administration comes back to the lower-level possibility of action in support of the junta in El Salvador. The present plan seems to be to double the amount of economic and military support to the junta. It is running at the rate of about $150 million a year. It is being pushed into the $300 million level.

Replacement planes for the six US helicopters destroyed in a rebel raid the week before were arriving during the past week in El Salvador. But the military initiative seems to have passed over from government forces to the rebels. And the idea of giving further military aid to the junta is unpopular on Capitol Hill and in important segments of American Opinion.

A formal call to end all US military assistance to the El Salvador junta was issued by the president of the Roman Catholic National Conference of Bishops, Archbishop John R. Roach of Minneapolis-St. Paul.

The above leads the Reagan administration into an unhappy dilemma. It wants to save the junta in El Salvador. It wants to topple Fidel Castro in Cuba. It wants to move against the Sandinista revolution in Nicaragua. It wants to reverse the drift in the Caribbean away from US-style anticommunism. It is pledged to do all that.

But when it starts down the road of military action, it runs into obstacles so serious that they are not being overcome. The prospect as seen among friendly diplomatic observers is for Castro survival, consolidation of the Marxist-oriented revolution in Nicaragua, and a long ugly and bloody civil war in El Salvador.

The amount of military aid which Congress and US public opinion will permit being sent to the junta in El Salvador will probably not be sufficient to give the junta a decisive victory. The junta may be able to hold the main cities for a long time, with US weapons. But the rebels will probably gain effective control of the hilltops and jungles and be able to carry on perhaps as long as the rebels in Afghanistan can carry on against Soviet weapons in that country.