There's nothing wrong with debating the legality of the Grenada invasion - or the duration of the American presence there. But it's a mistake to fix on these questions and ignore the one major and several minor opportunities created by the Grenada overturn. So far Washington seems to be doing so.
The major opportunity is a rare chance for a United States peace initiative on El Salvador-Nicaragua.
The minor opportunities involve a favorable atmosphere for improving on the languishing Caribbean Basin Initiative, and a chance to launch some creative ideas to improve the future lot of Grenadian workers.
First, a look at why Grenada has opened a window for a Central American peace initiative:
The dramatic success of the Marines in the south Caribbean (1) frees President Reagan to take a bold step on the peace front and (2) coincidentally puts pressure on Salvadorean rebels and the Sandinistas in Nicaragua to pay careful attention to any new Washington proposal.
From the White House perspective, Mr. Reagan cannot be seriously attacked by his right wing as being ''soft'' on Soviet-Castro forces if he makes a genuine new offer in Central America. Furthermore, such a move would fit just what some of the President's astute political advisers are seeking: visible evidence that Mr. Reagan is not a gunslinger but a leader who is tough only in the interest of advancing peaceful aims.
From the Sandinista-Salvador rebel perspective, it is a time to give serious consideration to any new version of an overall settlement plan. Among other factors, leftist leaders will not have missed polls showing a majority of Americans in support of the Grenada operation.
Nor will Central American leftists have missed the fact that many Latin American leaders, while publicly deploring the US invasion and voting against America at the UN, are privately saying (as a prominent Venezuelan leader did last week) that ''you did the right thing.''
It is possible that Mr. Reagan's Central American negotiator, former Sen. Richard Stone, is now testing the water for new momentum on the peace front. He left for a tour of the countries concerned shortly after the Grenada warfare ended. Certainly Mr. Reagan's other Central American prober, Henry Kissinger, is likely to see the window of opportunity created by Grenada.
Obviously, dramatic pressure will be needed from the White House if any real progress is to be made in the intractable El Salvador situation. Among other things, Mr. Reagan's military-diplomatic planners will have to be willing to lean very hard on the far right landowners and politicians of El Salvador. To stop their ''death squad'' approach to politics, it may be necessary to cut off military aid to them.
Ordinarily such a cutoff would open the President to accusations from his own direct-mail squads on the far right. But in the wake of Grenada and Pershing missile deployment, Mr. Reagan has a minimum vulnerability on that flank.
Meanwhile, the White House ought to be examining its opportunities on the Caribbean front.
First, there is probably a more favorable public climate in the US for the Caribbean Basin program, now that Grenada has shattered apathy toward that part of the neighborhood. That means a better attitude about Caribbean exports, and possibly about investment.
Washington planners need to avoid showering aid on Grenada while ignoring its neighbors. One inexpensive step to show the US cares about the whole area would be to appoint some bright young diplomatic representatives to the lesser islands in the Caribbean chain.
On Grenada itself it would help if US plans for repair of war damage included as much employment of jobless Grenadians and as little employment of Americans (except for technical experts) as possible. The Grenadian plea for the US to help complete the Cuban-built airstrip makes sense. So does employment of Grenadian construction workers to do the job. Ditto any plans to build new tourist facilities. (The airstrip makes sense largely in terms of more tourism, and the island is already short of the burned-down Holiday Inn on one of the prime beach-front sites outside St. George's.)
Washington should move promptly but modestly, consulting local leaders but not yielding to the temptation to create a lot of military ferroconcrete monuments. Fidel Castro has suffered his greatest setback in Latin America since Che Guevara's defeat in Bolivia. The US should take advantage by shifting emphasis from its military hand to its political-economic hand.