TO anyone trained in world affairs there was nothing new or startling about the speech US Secretary of Defense Caspar W. Weinberger made at the National Press Club in Washington last week. It was, to any history scholar, just ordinary and well-known common sense.
For example, he said the United States ''cannot assume unilaterally the role of world's defender.''
Consider these further admonitions: Don't commit combat forces overseas except in cases ''deemed vital to our national interest or that of our allies.'' Don't commit forces overseas without ''defined political and military objectives.'' And most important, don't commit US combat forces overseas without ''the support of the American people and their elected representatives in Congress.''
The important thing about the speech, therefore, is not what was said but by whom and at what time.
These rules for conduct of military affairs by a great power were stated by the defense secretary of the US at the end of the Reagan administration's fourth year in office. The speech was cleared through the White House. It was written, we are told, by Mr. Weinberger after months of thinking over the problem of where, when, and how a great power such as the US should or should not use its military power in combat.
It is ancient wisdom. The successful Pharaohs of Egypt understood such rules. Alexander the Great, the Caesars, both Julius and Augustus, practiced such rules. Napoleon violated them, and ended his career on the dreary little island of St. Helena. Hitler refused to recognize them and came to a stickier end.
The essential fact is that no one country, no matter how powerful, can run the whole earth by itself. The United States had almost the ability to do it for a brief span of time after World War II, when it alone possessed an undamaged industrial fabric and an absolute monopoly on nuclear weapons.
But that time receded into history as others developed modern industrial structures and their own nuclear weapons. The United States is still one of the only two superpowers on earth. But it cannot police the whole, without the consent of others, including the other superpower. To attempt to do so would be to commit the most dangerous mistake a great power can make: to commit beyond its resources.
It is ancient wisdom, but it was not the wisdom of the Reagan administration when its advance guard arrived in Washington four years ago and began to move into the offices of government.
The words of those days reflected a belief that it would be possible to regain that lost moment of history after World War II when the US could do almost anything it might have wanted to do; could, that is, had its people been willing to countenance new wars against former allies to make sure that no one could stand against the will of Washington.
Mr. Weinberger has been thinking well about some of the postures and aspirations of four years ago. Can the US ''roll back the iron curtain,'' liberate Poland, and Czechoslovakia, and Hungary, and bring down the ''evil'' regime that exists in Moscow? Mr. Weinberger knows the answers now. Perhaps it is easier to learn the answers at the Pentagon than at other points in Washington. It is there that men actually work out the expectable costs of overseas military operations in terms of human lives as well as in terms of weapons, ammunition, and dollars.
To learn the limits on military power is the beginning of wisdom in world affairs. The United States has been fortunate in one respect. It had a bitter lesson in Vietnam. But it was not fatal to the nation and its future, as was Napoleon's march to Moscow or Hitler's refusal to retreat at Stalingrad.
The reassuring thing about the Weinberger speech is that it means, if observed at the White House, that President Reagan will not during his second term commit American troops to combat overseas without first making sure that the objective is ''vital'' and that he has the consent of the political opposition and the support of a majority of the American people.
In other words, there is no second Vietnam in sight.