A high-level mission from the militant Marxist state of South Yemen quietly slipped out of Moscow last month with enough new pledges of Soviet military aid to alarm its neighbour, the kingdom of Saudi Arabia - the major source of oil for the industrialized democracies.

What makes this particularly significant, following the new Soviet-Cuban stranglehold on Ethiopia and the Horn of Africa, is the future prospect of Soviet-Cuban direct pressure on the vast Arabian peninsula. Real concern over such pressure would have been laughable a few short years ago, but it is laughable no longer in view of U.S. failure to know how to stop the Soviet-Cuban march through Africa.

The seeming impotence of the United States to deal with Soviet-Cuba, Inc., in Africa becomes a natural invitation for the Soviet-Cuban marauders to experiment elsewhere. Although there may be no immediate threat to Saudi Arabia or other oil-rich Arabian states on the Persian Gulf, the new Soviet military package for South Yemen is a [WORD ILLEGIBLE] of things to come.

It consists of what might be called Moscow's Grade B, economy-sized package 50 MIG-21s, of which 15 are believed already delivered; SA (surface to air missiles) 2s, 3s and 7s; plus an unknown number of modern T-62 tanks. Even if only the Grade B package, these weapons will dwarf what Moscow has previously sent to South Yemen - MIG-17s, T-54 and T-55 tanks. These earlier weapons have now been dispatched to Ethiopa for the Soviet-Cuban, Inc., war against Somalia.

With 500 to 900 Cubans now in South Yemen (mostly in the capital, the deep-water port of Aden) this tightening Soviet control increases the serious strategic threat to the Western democracies that use oil from the Persian Gulf. Most of that oil goes by tanker from the Gulf to the Red Sea. On one side of the entrance to the Red Sea (which is the gateway to the Suez Canal) is the Soviet-dominated Horn of Africa; on the other, just 35 miles across the Red Sea entrance, Soviet-controlled South Yemen.

Signs of Soviet-inspired political tightening in South Yemen are now too obvious to be in question. The titular head of the country has recently lost his preeminent position because he attempted to negotiate a political compromise with Saudi Arabia last summer. Since then he has been slowly eased out of power by the General Secretary of the Communist Party, a hardline, pro-Soviet loyalist.

For example, when the ruling Politburo asked the president what he and the Saudis had discussed after his experimental trip to Riyadh, he refused to respond on grounds that the talks were confidential and that "I am the president." He began to lose authority immediately and was replaced at the meeting of the anti-Egyptian rejectionist Arab States in Tripoli, following President Anwar Sadat's peace mission to Jerusalem, by the party boss.

The Soviet-Cuban lock on the oil passage into the Red Sea will not be complete until the Ethiopians put down the revolt in the Ethiopian province of Eritrea, a military operation that had to wait unitl the Somalis had been taken care of. In that civil war, South Yemen is now a clandestine combatant under Cuban tutelage.

It is believed - though not publicized - that a MIG-17 flown by a South Yemeni was shot down over Eritrea and the pilot's body recovered. Anti-Soviet Arabs further claim that this incident indicates that a full squadron of MIG-17s flown by South Yemeni pilots has probably been active in the conflict between Ethiopia and Eritrea.

Such intervention by Soviet "proxy forces" in local conflicts was a major target of President Carter's March 17 warning at Winston-Salem against Moscow's free-wheeling military politicies and the steady growth of Soviet power. He warned for the first time of what actually became apparent long ago - an erosion of support for U.S.-Soviet detente unless Moscow shows "restraint . . . in the projection of Soviet or proxy forces into other lands and continents."

Judging from the new weapons program Moscow has just agreed to for South Yemen, it is doubtful that Carter's warning will impose much restraint on the Russians. Rather, the United States may have to take more direct action. The buildup of a Soviet-Cuban, Inc., military force at the base of the oil-rich Arabian Peninsula and athwart the entrance to the Red Sea cannot be lightly regarded. A few years ago it would not have been tolerated.