Fanning outward over the desolate Desert of Death and upward into the rugged mountains, a massive Russian onslaught roared across Afghanistan just 10 weeks ago. In Washington, a harried Jimmy Carter responded by holing up in the Oval Office and reading stacks of foreign cables and intelligence reports.

Then, as the health of Yugoslavia's ailing Josip Tito began to wane, Carter called for reports on Yugoslavia. He also added to his late-night reading some reviews of Soviet strategy and history, including a study of the 1968 Soviet invasion of Czechoslovakia.

"Yugoslavia could be next," he suggested gloomily to subordinates.

To deter future Soviet aggression, the president dispatched naval task forces and amphibious landing units to the Indian Ocean. He also dropped grim hints that the United States would take the necessary steps to help the Yugoslavs repel a Soviet invasion. He assumed a calculated, heroic, pre-election posture -- ready to counter military moves that his strategists predicted the Soviets would not make.

The more likely danger, they told the president, would be an attempt to destabilize both the Persian Gulf states and the Yugoslav federation.

Classfied documents describe what is likely to happen in Yugoslavia during the post-Tito era. Without Tito's unifying presence, the new leadership will be collective but factional. Yet they will inherit a crack standing army of 250,000 and partisan forces numbering more than 1 million. Not only are these considered some of the best troops in Europe, but they have received special training to repulse an attack from the Soviet Union.

The Soviets, therefore, are more likely to use the same strategy that they have initiated in the Persian Gulf -- to destabilize, divide, decentralize and dismember Yugoslavia. They will seek to stir up old animosities and hostilities among the 22 million Croats, Bosnians, Macedonians, Montenegrins, Serbs, Slovenes and other ethnic factions.

To accomplish this, the Kremlin is expected to use Soviet sympathizers, known in Yugoslavia as Cominformists, who were driven underground or out of the country by Tito's anti-Soviet purges and show trials. The Cominformists have kept such a low profile, intelligence sources told my associates Bob Sherman and Dale Van Atta, that their exact strength cannot be measured.

Warns a top-secret analysis by the Central Intelligence Agency: "Official Yugoslav propaganda on the strength of the Cominformists generally maintains that the group is small, totally dependent on unnamed foreign -- but implicityly Soviet -- support. . . . In contrast to the public line, a closely held assessment circulating within the regime makes it clear that the threat from the Yugoslav Stalinists is considerably more serious."

The CIA appraisal indicates that the Cominformist underground is directed out of Kiev, Prague and ultimately Moscow. "The Cominformists are so well organized," states the study, "that they have been able to set up an illegal underground paper and to maintain caches of propaganda pamphlets in five different areas. Moreover, they are said to have an overall plan for disrupting Yugoslav stability, by force is necessary, and then calling in the Soviets for support."

Tito was able to govern his heterogeneous country by maintaining a loose federal system. This kept the fractious ethnic groups from indulging their natural inclinations to savage one another. But the Serb and Monenegrin nationalists still are disposed to domineer the smaller minorities.

The CIA strategists believe the Soviet sympathizers will encourage the nationalists to form a stronger federal government, which would tend to inflame the smoldering ethnic hostilities. Declares the secret analysis" "The Cominformist program is tailored to appeal to advocates of strong central authority, as opposed to the looser federal system Tito has created."

But the most destabilizing condition, which the Cominformists are sure to exploit, is a chaotic economy. Yugoslavia's inflation rate shot up to 27 percent last year, the highest in Europe. The unemployment rate, also europe's highest, hit 12 percent. Yet this figure didn't even account for the 700,000 Yugoslavs who went abroad looking for work.

There is imminent danger that Yugoslavia, without Tito, will thrash about and muddle into a chaotic bog. The Soviets may have no need to invade; they can merely wait in the wings.