Two different things

President Reagan's Central American plans are in trouble. Congress is unhappy about the continuing cost of the attempt to bolster the existing right-wing junta in El Salvador and even more unhappy about the supposedly, but no longer, covert campaign against the government of Nicaragua. US public opinion is skeptical and uneasy. Even the friendliest of Latin American countries disapprove. The West European allies also.

The troubles and the disapproval all spring, I submit, from confusing two different things - military security and political preferences. The troubles and the opposition in otherwise friendly quarters would largely disappear if the President would separate the two different features of his Central American concerns and approach them separately.

There is a serious problem of US military security.

The United States does not want and is not obliged to accept the presence in its own neighborhood of potentially unfriendly military forces strong enough to damage US military dominance in the area.

It is the proper responsibility of the President of the US to concern himself about such matters and to take such steps as may be necessary to prevent the intrusion into the US neighborhood of weapons of strategic and offensive nature.

In the Cuban missile crisis President John F. Kennedy did just that. When the Soviets put strategic missiles into Cuba which could reach the then unprotected central portion of the US, Mr. Kennedy mobilized the armed forces in the US, started their movement toward Cuba, and, in effect, told Nikita Khrushchev in Moscow, ''Either you take those missiles away or we will.''

There was no serious protest from friendly sources to the taking of the steps necessary to induce the withdrawal of those Soviet weapons from Cuba. Allies and friendly Latin countries would have accepted use of US forces in Cuba for the removal of those missiles, had that further step been necessary. The only serious criticism came from the political right, which wanted faster and more drastic action.

But President Kennedy did not go beyond requiring the withdrawal of the dangerous weapons. Indeed, he made what amounted to a deal with the Soviets. He agreed in substance that if the Soviets themselves removed the weapons he would allow the Cuban people to continue to have the government of their choice.

The US did not intervene in the internal domestic affairs of Cuba during the Cuban missile crisis, or since. Cuba still has a Marxist government which practices a form of nationalistic Marxism and it does cooperate with the Soviet Union in world affairs - to the continued distaste of the government in Washington. But the outside world, including the noncommunist countries of Latin America, approve of US tolerance and nonintervention.

The facts about the political coloration in Nicaragua are clear. The Marxist element in the leadership has increased, particularly since the beginning of the US destabilization campaign. The country may well turn into a full-fledged Marxist state. It probably will if the campaign against it continues.

And Nicaragua is supporting in El Salvador an insurgent movement which contains an influential Marxist element. If the insurgency won out the end result might easily be another Marxist state in Central America owing some degree of thanks to Nicaragua, to Cuba, and to the Soviet Union for its success.

Mr. Reagan regards this as part of the military security problem. His speech last week took for granted that if a country goes Marxist it automatically moves into the Soviet power orbit and becomes a prospective base for Soviet weapons to be used against the US.

This overlooks the fact that the world's most populous Marxist country, China , is still Marxist, claims to be more truly Marxist than is the Soviet Union, and is de facto a member of the anti-Soviet power bloc. Yugoslavia is another example of a Marxist country which pulled out of the Soviet orbit long ago, and does more trade with the West than with the Soviet bloc.

It may be distasteful to the US to have a country in its neighborhood go Marxist, but it need not be damaging. It would not be damaging if Washington simply said no to any placement of potentially offensive weapons in those countries, and enforced the ban firmly.

Such a policy might require a military operation to remove objectionable weapons. The use of US forces strictly for such a purpose would have almost universal acceptance, provided it did not involve interference in the domestic political coloration of the country involved.

The attempt to impose an American preference on the political choice of a people is what causes protest, opposition, and uneasiness. It is also the action which could most easily lead to a long-term US involvement as in Vietnam.

An enforced ban on offensive weapons would be free of the risk of long-term involvement. And it would be understood and accepted as proper by most countries and peoples.