Until quite recently, the Caribbean Sea seemed to be changing from an American lake into Fidel Castro's mare nostrum . From Puerto Rico to Panama, from Jamaica to Nicaragua, Cuban "advisers," propaganda and material aid flew unopposed, inciting fiery anti-Americianism and revolutionary radicalism. In spite of its patent failures -- submissiveness to the Soviet Union, miserable living standards, repression and discontent -- the "Cuban model" continued to be flaunted as the only valid solution for the people of the area. And behind the propaganda stands the ominous shaw of Cuba's military capacity.

As in many other parts of Latin America, progressive and democratic groups striving for a better future in the Caribbean, while opposing Marxism, are caught in an agonizing dilemma. While the left can count on immediate Cuban or Soviet support, and the right can rely on its traditional strength, social democrats and reformists can only appeal to the United States, a nation receding into a passive policy based on the assumption that non-imperialism means to renounce all initiatives. Under those conditions, any violent fight against repressive, corrupt governments implied the risk of having victory go to the radicals of the left.

The fate of Nicaragua seemed to confirm that gloomy prospect. Almost all Nicaraguans were anti-Somoza, many fought with the Sandinistas, few were socialists. Somoza's criminal stubbornness and Washington's fear of involvement contributed to a polarization that favored the Sandinistas. With no other alternative in sight, the Sandinistas eventually won. After victory, the revolutionary momentum continued to favor the radical few. Moderate revolutionaries and democratic elements appear to be losing ground to the better-organized pro-Cuban group.

At the same time, a wave of strikes hit Costa Rica. And poor, overpopulated El Salvador, where infamous oligarchy had systematically crushed every dream of reform, seemed to be the next target. The domino theory appeared to be working in Central America.

A new development, though, has atlered the situation. Jolted by the "discovery" of Soviet troops in Cuba and the extent of Castro's penetration in the Caribbean, the United States seems to be ready to meet the challenge. iPerhaps the old ideal of the Alliance for Progress is also being revived: to check the communist threat by helping economic progress. The stiffening of American policy toward Cuba produced immediate results in the Caribbean. On the island of Dominica, two ministers have been dismissed for being communists. Opposition to Prime Minister Michael Manley's socialist policies has gained strength in Jamaica. In El Salvador, a militry coup of young officers has established a progressive government of civilians and military men. They have announced a program of social and economic reforms, amnesty, elections and respect for human rights. So far the new government has received the support of many political groups, and the cautious approval of Archbishop Oscar Romero, a gallant champion of human rights, and perhaps the most popular figure in El Salvador.

The problems of El Salvador are awesome, and the enemies of the government are very powerful. Already the leftist elements of the Popular Revolutionary Block, aware that reforms are the worst enemies of revolution, have proclaimed their violent opposition. Less vocal but almost as dangerous, the right is digging the usual trenches of delays, rumors and economic panic. And even in the United States, those who confuse change with communism are raising the specter of "another Nicaragua" in Central America.

Actually, this new government has accomplished in El Salvador precisely what could not be done in Nicaragua: the opening of a third alternative, possibility of change in a democratic framework. If successful, this effort could demonstrate that in an underdeveloped country the road to social justice does not have to fall into the iron mirage of Marxism. If the Sandinistas choose to pay the terrible price of the socialist path, El Salvador could become an example of the superior capacity of democracy to accomplish social progress with political freedom.

El Salvador has attracted continental attention. Many progressive groups in Latin American are looking to Mexico, Venezuela and a few other Latin American democracies and, above all, the United States for help. They expect a true committment, a sincere effort to help this feeble, fragile government, which embodies such unexpected possibilities and which is struggling against so many enemies of the left and the right. Teetering toward Cuba, the Sandinistas have, nevertheless, received American support. The new government of El Salvador certainly deserves better. If for lack of external aid it fails, the third alternative will fail with it, leaving only the ankylotic right to stand before the revolutionary left. Then the domino theory will probably be played out in Central America.