John Bartlow Martin would have us believe that neither the Cubans nor the Soviets have anything to gain from assisting the Salvadoran rebels ("Listen to Lopez Portillo," Topic A, March 14). He finds the idea of "evil conspirators" to be "laughable." Really? Well, keep on laughing.

Our intelligence has a very good idea of what the guerrillas are getting from their friends abroad, just as we knew what the Sandinistas were getting through Cuba, Panama and their staging areas in Costa Rica. We knew the types and numbers of heavy ordnance received, the aircraft involved, the personnel on board and a good deal more, including the fact that some of those weapons were shipped from Lebanon. So who's kidding whom?

One has to wonder, then, why? Why this passionate effort at political alchemy? There is no Cuban or Soviet interest and involvement in Central America? There are no Marxist revolutionaries in this hemisphere? The guerrillas are just harmless agrarian reformers who wish us well? It must be so, because a year ago our former ambassador to El Salvador told a congressional committee that the guerrillas there were no threat but that U.S. military assistance to the government would be! Curious.

Let's examine a few other myths. Revolution, guerrillas and terrorism are the product of poverty and injustice. That's simple enough. That's why terrorists like the Red Army Faction, the Red Brigades and the Japanese Red Army exist in poverty-stricken and totalitarian countries like West Germany, Italy and Japan. Nor do such old- fashioned motives as expansionist ambitions or political adventurism exist in Moscow or Havana.

Another myth: it used to be a reasonable assumption that you didn't leap blindly from a bad situation to a worse one. Not anymore. Our human rights champions bemoan the shortcomings of a shah, decry the "brutality" of the Latin American military, express outrage at the lack of democracy in right-wing governments and demand that the United States halt all dealings with them on moral grounds. Where are the critics of the shah today? Are they commenting on the flowering of democracy in Iran, the evenhanded dispensation of justice, the enhanced role of women? No, they are too busy ferreting out injustice in El Salvador and Guatemala.

None of this is intended to argue that there is justice and decency everywhere in Central America. But does anyone seriously believe we can influence or wean away Marxist ideologists from their objectives? Nor can anyone excuse the steady stream of self-inflicted wounds caused by the ineptness of our own government. The State Department shot itself in the foot again with the captured Nicaraguan "witness" episode, but that doesn't change the basic facts. Neither the guerrillas in El Salvador nor the Sandinista regime in Nicaragua would exist without massive assistance, training and material support from Cuba, et al. Anyone who can't see that chooses to be blind.

One final point: the same "freedom-loving" forces in Central America who talk like Marxist revolutionaries, act like Marxist revolutionaries and have lived all their lives as Marxist revolutionaries have miraculously transformed themselves into the "heroes" of this tragedy. These "sweethearts" are the same terrorists who have kidnapped and murdered foreign businessmen and diplomats, assassinated police and government officials, kidnapped and murdered their family members, intimidated judges and prosecutors--all in the name of revolutionary "justice."

A political "solution" that incorporates the guerrillas is the victory they want and can't win on the battlefield. Let the proponents of that "solution" name one country and one time when moderates sat down to reason with Marxist revolutionaries and survived! It won't work now, and the proponents know it. The irony is that, when the worst happens, many of them will be in the forefront demanding to know who was responsible for the debacle.