President Reagan and his lieutenants were categorical about not sending United States troops into combat when they first started out on the road of intervention in El Salvador and Nicaragua.
As of today, so far as is known publicly, US military advisers and instructors are not going into active combat.
But the assertions about US troops in Central America have gradually softened. The current line is that ''we do not see any contingency under which American troops would be used or needed in Central America.''
That was US Secretary of Defense Caspar Weinberger on the CBS program ''Meet the Press.''
He added that ''we have to take each situation as it comes.''
So ''under no circumstances'' has been modified to leave open a possibility depending on need.
Which is a familiar story. First, just advisers and instructors. Then the advisers and instructors follow their pupils into combat to see how they do under actual battle conditions. Then an instructor finds himself showing them how to do it, etc., etc.
Since the last big war (World War II) there have been many examples of military intervention by one country in the affairs of another. The record would indicate that it is a risky business, even for one of the superpowers.
The most successful such interventions have been the Soviet suppression of dissent in Hungary, Czechoslovakia, and Poland (by proxy, using the Polish army). The key to success in those cases was the use of overwhelming force, suddenly and unexpectedly.
But the Soviet invasion of Afghanistan, although equally sudden and in substantial force (although not as overwhelming as in the other three cases), has bogged down. About 100,000 Soviet troops have been there for three and a half years. Resistance is still vigorous. Moscow is supposedly searching for a way out without losing face or superpower credibility.
Israel has run into trouble with its invasion of Lebanon. A quick reach for security on the northern frontier has turned into a bloodletting with no end yet in sight.
US intervention in Korea goes down as a partial success. South Korea was saved from communism and is today one of the most prosperous and successful of the newly industrialized countries. But the attempt in the second phase of that war to ''liberate'' North Korea ended in a military disaster. Gen. Douglas MacArthur launched his big winter offensive on the assumption that the Chinese would not dare to enter the battle against him. They did enter, in massive force. They smothered his offensive. Korea has been divided ever since.
The worst failure was US intervention in Vietnam. The US intervened massively. At the peak US troop strength in Vietnam reached the half million mark. The effort was sustained for eight years. It ended in failure.
Argentina's invasion of the Falklands was worse, for the Argentines, in one respect. Defeat was ignominious. The Argentines were fighting just off their own coast, had numerical superiority, and could provide their invading troops with land-based air cover.
But at least it was over quickly. There was nothing like the sustained effort on a large scale which the US experienced in Vietnam.
Of course the reverse holds. The British, with smaller numbers but better weapons and higher standards of professionalism, won a brilliant victory.
Trial by arms is a risky business.
British success in the Falklands was by no means a sure thing.
When the Israelis surged into Lebanon a year ago they had much reason to expect quick, decisive, and relatively inexpensive success. Would they have launched the offensive had they foreseen that a year later they would be taking casualties almost every day, and now they can neither go forward nor get out?
Lyndon Johnson would certainly never have sent half a million American boys into the jungles of Southeast Asia had he been able to foresee those names carved in their thousands on the black wall in Potomac Park in Washington. He thought it would all be over quickly, and cheaply.
The point of all this is that one can never be certain about the outcome of a battle. No matter how favorable the odds may seem, accidents can occur. That is why, down through the ages from the days of ancient Rome, the textbooks in military academies have always advised commanders to seek their ends short of battle if they possibly can.