This has been the week when Ronald Reagan had to learn, the hard way, one of the old classic rules about military intervention by great powers in the internal affairs of small countries.
The old rule is that if you are going to do it, then do it with decisive force, as quickly as possible, and don't worry too much about the legalities. If it is not a quick success, chances are that the intervener will get bogged down in endless and expensive troubles.
The United States invasion of Grenada is an example of the effective way to do these things. The US Marines went in with decisive force and took all their first-day assignments, quickly and with light casualties. The quick and successful nature of that operation contrasted with the difficulties with the two other ongoing US military interventions - in Lebanon and Central America.
The death count of marines from last Sunday's bombing of their barracks outside Beirut reached 221, with some still missing, presumably in the rubble.
US intervention in Lebanon this time around began on Aug. 25, 1982. Some 800 were sent in to help police the Palestine Liberation Organization's withdrawal from Lebanon. The President promised at the time that they would remain for only 30 days.
Now, 14 months later, more Marine reinforcements are arriving, the casualty toll has become a politically important issue at home, and the top-level foreign policy managers of the government in Washington are groping, so far inconclusively, for some way of either protecting the marines or getting them out.
The Lebanon intervention was relatively small. The place is a third of the way around the world from home. The mission was vaguely defined as ''peacekeeping.'' No end is in sight.
Precise numbers of US armed forces in Honduras this week are not available. ''Maneuvers'' are going on intermittently. The purpose is primarily support of counterrevolutionary forces staging raids into Nicaragua from Honduras. The government in Nicaragua has been hurt by the counterrevolutionaries, but the US role, supposedly ''covert,'' has long since ceased to be covert.
US public opinion has inclined increasingly against this inconclusive and less forthright type of intervention.
The government in Nicaragua is recognized as being legal by the Organization of American States, and by the UN. The US is in fact at war with that government , but only covertly. There is still a US diplomatic mission in Managua, the capital of Nicaragua.
The operation in Honduras has failed of one of its purposes, which was to hobble the leftist revolutionaries in El Salvador. The insurgents are still in the field there. The government of El Salvador, which enjoys substantial US support, has been unable to end the work of rightist ''death squads,'' whose activities are now threatening continuation of US support.
US intervention in the Salvadorean-Nicaraguan situation dates from early Reagan days. It has become broader and more expensive by the year. The largest concentration of US armed forces ever known in Central America is under way. No end is in sight.
The Grenada operation had the merit of being quick - and quickly decisive. There was no doubt that the Marines once more had landed in a Caribbean country. It was about the 61st or 62nd time - depending on which source you use and how many of the landings should be counted. As usual, ''the situation is in hand,'' although some Cubans seemed to be still resisting when we went to press.
The expectation, probably realizable, is that the resident governor general (Grenada belongs to the Commonwealth) will be able to bring respect to the affair by arranging for elections and the setting up of a new government more acceptable than was the old one to Grenada's Caribbean neighbors, and to the US. The Marines could go home quickly. It could end as an almost perfect exercise in ''gunboat diplomacy.''
But the Grenada operation had the demerit of not having been sold to the British in advance. British Prime Minister Thatcher considered it to have been ''unwise.'' Probably she was disturbed by the implication that it is proper for any American state to intervene in others' affairs. She presumably had the Falkland Islands in mind. Argentina has not renounced its claim to those islands , despite last year's ignominious failure to take them by force.
And the use of US armed forces in Latin countries is generally unpopular in those countries, even among the more conservative populace.
Perhaps the most intriguing angle to this reflexive use of military power by the Reagan administration is its possible effect on US political patterns. Mr. Reagan's domestic policies have so far been relatively successful, but the Grenada affair is his first successful (we presume) foreign adventure. Its very success highlights the nonsuccess of his major foreign policy initiatives in the Middle East and Central America.