Why not send troops?

The more I read, and read about, President Reagan's Central American policies , the more I am inclined to the view that he has made a mistake in arguing a case for national security and then declaring that: ''There is no thought of sending American troops to Central America.''

What are the armed forces of the United States for if not to protect the US against any threat to the national security of the US?

Mr. Reagan has stated the existence of a security threat in strong terms. He invokes President Harry Truman and the Truman Doctrine. He recognizes that the countries involved (Nicaragua and El Salvador) ''are smaller than the nations that prompted President Truman's message'' (the message which preceded US aid to Greece and Turkey).

''But the political and strategic stakes are the same,'' he asserted in his April 27 message to the Congress.

Well, President Truman felt that the threat to Greece and Turkey justified sending to those countries not only weapons and money but also armed forces. The US military presence in the Eastern Mediterranean dates from the Truman Doctrine of March 6, 1947. There have been substantial US armed forces in the area ever since, including naval and air bases.

Is the threat to US security in Central America comparable? If it is, then why not use the armed forces of the US to counter the threat, or at least leave open the possibility of using these forces?

In all logic a declaration against sending US combat forces to the area downgrades the threat. It can also give a green light to present or prospective opponents.

There is a school of thought that the invasion of South Korea in 1950 was triggered by the withdrawal of US troops from Korea and by several statements by high US officials that South Korea lay outside the defense perimeter of the US.

In the 1976 presidential campaign debates President Gerald Ford raised the point against Democratic candidate Jimmy Carter. Mr. Carter had stated that if he were President he would not go to war in Yugoslavia ''even if the Soviet Union sent in troops.'' President Ford, in rebuttal, referred to the Korean war and said, ''It is unwise for a president to signal in advance what options he might exercise if any international problem arose.''

There is an ''international problem'' in Central America. The US government is unhappy about current trends in both El Salvador and Nicaragua. The President wants Congress to fund a program of military and economic support for the government in El Salvador and for counterrevolutionaries in the field fighting against the troops of the government of Nicaragua.

But he has committed himself against using US combat troops in his approach to the situation.

This has two damaging consequences.

First, it tells both the rebels in El Salvador and the government in Nicaragua that they need not be concerned about the possibility of facing US combat forces. The President in Washington has tied his hands and reduced his options. He is going into action only with such funds and weapons as Congress will authorize him to use. The enemy can raise the ante and Mr. Reagan will not be able to match them.

This in turn has a second adverse consequence. If there is a true security threat, and if it is not going to be matched even to the point of US combat troops, then the junta in El Salvador and the insurgents in Nicaragua enjoy almost a blank check from Washington. The defense of US security is in their hands. They are indispensable to Mr. Reagan's purposes. They can do almost anything they like without losing their indispensability.

Mr. Reagan says he is going to press upon both the El Salvador junta and on the insurgents in Nicaragua the importance of ''democracy, reform, and human freedom.'' But if they are indispensable, they will be under little sense of urgency about these three values.

If there is a true threat to the national security of the US then, at the very least, the possibility should be left open of using US combat troops to counter the threat.

If there is no thought of using US combat troops, then the White House cannot consider the threat to be even remotely comparable to the danger which prompted the Truman Doctrine speech and anchored the armed forces of the US back into the Eastern Mediterranean.