It was all there -- tropical moon, secluded beach, pina coladas, strains of Caribbean rhythyms, and, at the end, captured communist documents.

I feel as if I've been in a time warp. After clearing customs in Miami, ending the first trip back to the Dominican Republic since Lyndon Johnson dispatched the Marines to Santo Domingo 16 years ago this spring, I picked up the first American newspaper seen in days. It bristled with old-new Page One news. Our government has uncovered a communist conspiracy to overthrow a banana republic regime.

Everything about it was familiar.

The official U.S. language: "Marxist-Leninist guerrillas [are] now fighting to overthrow the established government." The "definitive" evidence: f"drawn from captured guerrilla documents and war materiel and corroborated by intelligence reports, [it] underscores the central role played by Cuba and other communist countries . . . in the political unification, military direction and arming of insurgent forces." The spelled-out threat: "Their objective . . . is to bring about -- at little cost to themselves -- the overthrow of the established government and the imposition of a communist regime in defiance of the will of the . . . people." The case for possible U.S. action: a State Department "white paper," filling eight full columns in The New York Times, that details the "clandestine military support given by the Soviet Union, Cuba and their communist allies" to rebel forces in El Salvador.

In case you've forgotten, we've been through this one before. The question for today involves what lessons we've learned.

When critics questioned his dispatch of American troops to foreign lands that fateful spring of 1965 -- first to Santo Domingo and then in ever greater numbers to Saigon -- Lyndon Johnson responded with a burst of evangelical patriotism. "I have seen the glory of art and architecture," he said, recalling a declamation from his youth. "I have seen the sun rise on Mont Blanc. But the most beautiful vision that these eyes ever beheld was the flag of my country in a foreign land."

Now, after so many years of national failures and frustrations, that kind of old-fashioned jingoistic flag-waving is IN again -- with fervor. Call it what you will, the New Nationalism, the New Patriotism, the New Beginning, or (thanks, Time Inc.) the American Renewal, the new Reagan administration is reasserting American power and attempting to rekindle American pride in country. Few would deny some such effort is needed; we have been wallowing in a self-inflicted state of despair for too long. But we also need a measure of New Realism to go along with our new flexing of muscles and minting of macho slogans.

Let it be said immediately that El Salvador will not be another Vietnam. The country will not tolerate being bled white by another foolhardy adventure in the jungles of some scarcely known nation that poses no threat at all to legitimate American interests -- and the stakes and conditions in El Salvador are infinitely different from Vietnam. And let it also be said that a critical task for the Reagan administration is to define U.S. international goals and unmistakably make clear to the world that America intends to exercise its power when necessary. Self-interest may be selfish, but it is both wiser and more practical than a noble-sounding but unworkable human rights policy.

But saying this does not make sensible the administration's calculated beating of drums or rattling of sabers. What's at issue here is nothing less than understanding the series of mistakes that led us to disaster beginning at the Bay of Pigs in Cuba 20 years ago, continuing on with the landing of 21,000 American soldiers and Marines in Santo Domingo, and culminating in our hemorrhage in Southeast Asia. The question involves the nature of communism, how much of a threat it represents, and what the American reaction should be.It involves the distinctions between leftist revolutions and communist ones. It involves remembering when a decision to intervene becomes an unwanted declaration of war -- and that it is easier to go in than to get out. And, more than anything else, it involves the necessity to be extremely discriminative in the application of American force.

El Salvador, in particular, presents a classic case of a people who are being crushed between right-wing repression and left-wing terrorism. It also should be a reminder to U.S. policy makers of the reality of revolutions. They are not won by guns alone -- as that administration white paper dramatically (and no doubt unintentionally) demonstrates. Among the weapons being furnished the Salvadoran rebels, it says, are "60 tons of captured U.S. arms sent from Vietnam."

Are the Reagan people reading their own lines? But then in writing this I realize I'm probably guilty of another old sin dating back to seeing those earlier U.S. Caribbean adventures -- being soft on communism.

Note: Among the outpouring of mail and calls in response to a recent column about my problems with the postal "service" was a letter -- promptly delivered -- from Washington's new postmaster, John R. Cochran. He says: "I want to pledge to you and your readers that I will do everything in my power to see that Washingtonians receive the good, reliable, courteous mail service that they deserve and have every right to expect." He adds: "Discourtesy, rudeness or similar behavior by postal workers in their dealings with the public are unacceptable and are not condoned by postal management or by the majority of postal employes." And, he says, a complaint mechanism to which I referred already exists in the form of a "Consumer Service Card," available at the main post office, postal stations, of from individual carriers. "I urge customers to use this card," he says, "or to call the Customer Services complaints officer at 523-2226. I feel so strongly about this that I am having additional telephone lines installed. When installation is completed, additional personnel will be designated to handle complaint calls and to improve the process of resolving customer mail problems."

Right on, Mr. Postmaster. You'll be hearing from us.