Beirut — Just over one year ago, a key American official was pressed on why the Russians had not more openly challenged US dominance in the Middle East, the area of the most intense running rivalry between the superpowers.
''They are probably waiting for it to fall apart for us,'' he speculated. ''And then they will try to step in and pick up the pieces.''
The waiting game is now over for the Russians.
The Reagan administration's recent fumbling of its Middle East policy, both in Lebanon and the Arab-Israeli conflict, has coincided with a noticeable increase in Soviet activity in the region, where suddenly Moscow's envoys are both more visible and more welcome. Many diplomats feel the potential for a Soviet role has not been so great since 1973, the turning point for a subsequent decline in influence. (For other Mideast events, see Page 9.)
Yet envoys from the Eastern and Western blocs and the Arab world point out that Moscow's new prominence and prestige are more by default than as the result of any bold initiative.
''The Soviets really inherited the situation. The Americans gave it to them on a silver platter, not just Lebanon, but many countries shifted their views as the US was increasingly seen as an unreliable ally,'' a Western source said.
Beirut's English-language Daily Star said: ''By far the biggest losers have been those in the Middle East who put their trust in the US and who sought in vain to reconcile contradictions in American policy. Lebanon counts its dead. Moderate Palestinians see hopes vanish of a negotiated peace. King Hussein fears for the safety of his state. Gulf rulers wonder whether to hedge their bets on the US as a guarantor of their security.''
Anger, frustration, and disappointment with US actions over the past 18 months were repeatedly cited as the main reasons for the shift. ''It isn't so much that the Arabs prefer the Russians, but a lot of them feel they now must either have more balance in their foreign relations or turn to someone with the capability of follow-through, be it on the issue of weaponry or political backing,'' an Arab envoy explained.
The new warmth toward the Soviets was symbolized during the visit last week to Lebanon of a Soviet delegation led by Karin Brutents, a high-level member of the Communist Party Central Committee. Just two months ago, President Amin Gemayel had warned that if US Marines were withdrawn, ''There would not be a new president to replace Amin Gemayel, but a revolutionary council under Soviet control, or chaos.''
Last week Mr. Gemayel, his prime minister, and foreign minister all held widely covered separate sessions with Mr. Brutents, who on a visit just a year earlier was basically ignored by Lebanese officialdom. And since the de facto end of the US political initiative, the Soviet ambassador's advice has been more frequently sought at Baabda Presidential Palace.
But the Russians have been even more active on other fronts over the past two months.
In Syria, Moscow's strongest Mideast ally, relations were strengthened after last month's visit by Geidar Aliyev, deputy premier and member of the Politburo.
Although there was no official communique on new aid, it was widely reported in the Arab world's press that the Soviets had agreed to sell Damascus MIG-29 and MIG-31 warplanes, their most sophisticated aircraft, as well as an electronic defense system to match US supply to Israel of a satellite-aided communications network. However, the two nations did announce plans for the Soviets to help build Syria's first nuclear power plant.
In Egypt, the US's closest ally, President Hosni Mubarak last week announced that full diplomatic relations with Moscow would be restored ''soon.'' Trade over the past year has already soared. And last month Usama al Baz, Mr. Mubarak's top political adviser, said the Soviet Union could not be excluded from any broad Mideast peace effort.
In Jordan, King Hussein on Saturday told a visiting American group that the Soviet Union had a ''strong presence'' in the region, adding that any attempt to settle the 35-year Arab-Israeli conflict would be greatly enhanced by Russian participation. This follows a statement last month from the monarch that the US had lost its credibility as a mediator in the Middle East.
On arms, the King has also shown willingness to turn to the USSR. In March he told a US television network he was prepared to ''look anywhere'' if the US cancelled its offer to sell 1,600 Stinger missiles - which the Reagan administration subsequently did after pressure from Congress.
In Iraq, the Soviets appear to have gained major ground. Although Moscow was initially neutral when the Gulf war broke out in 1980, it has recently begun to openly side with Baghdad. Sources close to the Soviet Embassy in Beirut said Moscow has ''very actively'' resumed large-scale arms supplies to Saddam Hussein.
With the Palestine Liberation Organization, the Soviets have heightened their attempts to heal the rift between radicals and moderates. They have also attempted to end the feud between chairman Yasser Arafat and the Syrian leadership. Soviet Deputy Premier Aliyev reportedly exerted strong pressure on Syria to end backing to rebels demanding Arafat's ouster.
In Libya, another strong Soviet ally, the regime of Col. Muammar Qaddafi will receive another boost this week during the visit of a Soviet squadron of warships.
Even in the conservative Gulf sheikhdoms, the Soviets appear to be making minor headway, even though they have formal relations with only Kuwait and South Yemen.
There is growing feeling in the Gulf among Western diplomats that Riyadh's anger at the US could lead to quietly allowing stronger East bloc connections with the Saudis' sister states as a prelude to direct ties.
Ironically, Arab and East bloc sources suggest that the new Soviet prominence is at least in part the result of a US policy based on fear of a Soviet role in the region. As the Daily Star commented last week:
''If the US wants peace in the Middle East, why does the Reagan administration stress superpower rivalry in a region where this had until then been of secondary importance? The result: a self-fulfilling prophecy that elbows aside the real forces and interests in action in the Middle East.''