Force and friends

SECRETARY of Defense Weinberger, in his speech to the National Press Club last week, set forth a sensible doctrine regarding the use of United States armed force. His remarks have significant implications for our foreign relations.

His references to the use of the military as a ''last resort'' when ''vital'' interests are involved, to the importance of ''clearly defined'' objectives, and to a ''reasonable assurance'' of public and congressional support should be understood by leaders in other countries who may expect the US military to come to the aid of their causes. These principles should apply, as well, when we are tempted to encourage others to use armed force.

We have, since World War II, given the impression to friendly foreign rulers that we would rescue them if they should be threatened. Statements that stressed ''friendship'' and ''support'' undoubtedly suggested to earlier rulers of Iraq, Libya, Ethiopia, and Iran that the US attached such importance to their leadership that we would undertake extraordinary measures to protect them. When a crunch came, we were found wanting; the capacity and the necessary consensus were lacking.

In some cases, our commitments have been more specific - through presidential letters or treaties. Saudi Arabia and the Philippines are examples. In neither case is there a specific commitment to the use of force, but our officials have frequently suggested that we believe these countries to be part of our vital interests.

The case for honesty with those whom we may encourage to undertake armed combat, particularly through covert means, is compelling for practical and moral reasons.

Whether there were signs from Washington that encouraged Israel to invade Lebanon is still debated. There were, perhaps, too few signs that we would disapprove. Given the inevitable likelihood that we would be identified with this action, too little effort was made to decide whether there were alternatives, whether the objectives were well defined, or whether there was a reasonable assurance of support in this country.

The case is clearer in Nicaragua. There is little doubt that we encouraged the contras' formation and action. Whether we had more limited objectives than the contra leaders is beside the point; there was seemingly no agreement on objectives. If so, it was naive of us to think that our support would not be seen as a green light to the contras to seek the overthrow of the Sandinistas.

When we encourage others to expect US support or to undertake armed struggle, we also have a moral obligation. We are asking people to take political and physical risks with the expectation that we will support them fully. Too often we have, in a crisis, been unable to do so.

There are those who will ask, legitimately, ''Does this mean the US is powerless to help its friends and defeat its enemies?'' ''Must we sit by and see Soviet support for local wars and revolutions, powerless to help our allies?''

The answers are not, necessarily, ''yes.'' But any responses must recognize that there are no secrets in our democracy; if our side will not talk of secret or covert efforts, others will. There is scarcely a significant quiet commitment or a clandestine military action that has not been revealed and, where consensus did not exist, has not become a matter of national debate. Events have demonstrated that where strong indigenous support for an ousted regime did not exist, revolutions could not be reversed by outside efforts. Those whom we would support have been exposed and have suffered severely. Where there is doubt about the strength of our friends or the depth of support for an action in our country , we are wrong to encourage those we cannot sustain.

Some years ago, after the fall of Saigon, a Vietnamese diplomat was asked, ''What would you advise another nation seeking the support of the US?''

He replied, ''I would suggest they think twice.''

When pressed to explain his answer, he added, ''I would advise another nation never to seek the support of the United States without thoroughly understanding the American political system.''

This is sound counsel.