A secret agreement between the Soviet Communist Party and the Sandinista Marxist Party of Nicaragua, signed in Moscow in late March, points to major new Soviet intervention and a possible Nicaragua-Cuban military accord in the increasingly hostile Caribbean back yard of the United States.

"The Soviet government would be unlikely to sign a party-to-party agreement with a foreign country unless Moscow regarded that country as a reliable, long-term partner," a leading Latin American authority told us.

The secret agreement between the two ruling parties is regarded by non-communist Latin nations as a dangerous Soviet intrusion into the Central American heartland. It appears aimed at planting Soviet influence and arms smack on the bridge between the Americas, flanked by what used to be the peaceful lake of the Caribbean but what is now becoming an alien sea dominated by the Soviet dependency of Cuba.

What is actually contained in the secret party-to-party agreement is not known here or by the governments of principal Latin American countries. There is reason to suspect, however, that Moscow is encouraging a military link between communist Cuba and the Sandinistas, who toppled Anastasio Somoza's right-wing dictatorship last year. That link would guarantee money, arms and supplies for Nicaragua. If so, Moscow would gain automatic entree to the struggling Sandinista regime through a kindly intermediary -- Cuba's Fidel Castro. Without Castro, the Marxist revolution in strategically placed Nicaragua could not have succeeded last summer.

The Soviet plan is to communize Central America and, using that land bridge as a dagger pointing north and south, extend its influence onto the two continents. What is surprising to top policy-makers in the Carter administration is the speed with which this Soviet campaign is progressing, and the increasing boldness of the men in Moscow and their agents in Havana.

U.S. intelligence sources are now certain, for example, that Castro is upgrading his estimated 144 SAM2 anti-aircraft missile sites on Cuba. Now single-stage rockets, the Soviet-made SAM2 is acquiring three new boosters. Some U.S. officials believe the purpose is to provide better air defenses for possible U.S. retaliation against a Soviet-Cuban decision to redeploy nuclear weapons in Cuba some time in the future.

After the 1962 Cuban missile crisis, the Russians promised never again to move offensive nuclear weapons into Cuba. But the Soviet invasion of Afghanistan broke the back of detente and U.S.-Cuban relations are at a new low -- poisoned in part by Castro's deadly missionary work in Central America. Castro is clearly preparing for new confrontation with Washington.

He now has at least 2,000 Cuban agents in Nicaragua, including military advisers who control a small training camp at Esteli and a fortified air facility at Montelemar. Some U.S. officials claim that although the Sandinista regime is controlled today by pro-Castro Marxists, a political power struggle continues within the revolutionary group.

If so, there was no sign of it during the four-day, late-March Moscow summit between the Sandinistas and two of the highest Soviet Leaders, including Politburo member A.P. Kirilenko and Boris Ponomarev, Moscow's chief liaison with foreign Communist parties. a

The communique describing that session outlined several routine agreements -- widely published here and in Latin America -- on consular, trade, air traffic and other relations. The Soviet version of the communique then stated in the next-to-last paragraph: "A plan of ties" between the Soviet Communist Party and the Sandinistas "for 1980-1981 was signed."

It is that bare-boned agreement that U.S. and Latin authorities believe points toward a military arrangement of some kind between Nicaragua and Cuba, with the Soviets as middleman/broker. It was signed by a leading member of the Sandinista junta and by ministers of internal affairs (Nicaragua's top cop), defense and economic planning -- the revolutionary regime's high command.

The commitment to Soviet world policy by these four Sandinistas is breathtaking: denunciation of the U.S.-NATO decision to upgrade NATO's nuclear arms, defense of the Soviet invasion of Afghanistan (later gently repudiated by a member of the junta) and "renunciation of the use of force" -- notwithstanding Afghanistan.

Such rhetoric pales alongside the possibility of a made-in-Moscow military alliance between Cuba and Nicaragua. While President Carter fumes over American hostages in Tehran and laments Afghanistan, aggression of a more ominous character is being practiced in our own back yard. a