The imminent visit to Damascus by a top Kremlin figure comes at a time when the crisis in Lebanon - and American setbacks there - have helped bring Soviet and Syrian interests into -uncommonly close harmony.
The planned visit by Politburo member Geidar Aliyev also follows an unprecedentedly large-scale Soviet move to resupply and modernize the Syrian military, amid a confluence of Soviet and Syrian concerns in the Mideast as a whole.
Despite the signing in 1980 of a formal Soviet-Syrian friendship treaty, relations between Moscow and Damascus have not always been as simple or tranquil as is routinely assumed in the West. The most recent instance of strain came only a few months ago, over Syrian support for a mutiny within Palestinian leader Yasser Arafat's Al-Fatah guerrilla group.
Indeed, Syria's military involvement in Lebanon has on various occasions proved a source of tension between Damascus and Moscow. Although initial Soviet opposition to the Syrians' dispatch of troops to Lebanon in 1976 has since given way to something between acquiescence and support, Soviet officials still issue occasional reminders that they do not view the military coordination commitments in the 1980 treaty as applying to Syrian forces in Lebanon.
This is Soviet shorthand for a concern not to be dragged into a Mideast confrontation at a time, or on a scale, not of the Kremlin's choosing.
Syrian President Hafez Assad, too, has been consistently leery of forfeiting Mideast maneuvering prerogatives through too close an embrace with the Kremlin. For some time before 1980, Mr. Assad had resisted the idea of a formal treaty with Moscow - in favor of an acrobatic brand of ''nonalignment'' leaving open the option of a US-mediated return to Syria of the Golan Heights, captured by Israel in the 1967 Mideast war.
It was only after the late Egyptian President Anwar Sadat's go-it-alone treaty with Israel that an alarmed Syria formalized the tie with Moscow. ''Ultimately, I would not say we trust the Syrians,'' remarked one Mideast specialist in Moscow to this reporter and two visiting US journalists even as the Soviets were rearming Syria on a large scale in 1983.
But if the element of distrust survives - on both sides - it has been increasingly overshadowed of late by a rare unity of interests in the Mideast, and notably in Lebanon.
The announcement that Soviet politician Aliyev, promoted to a parallel post of first deputy premier in the Soviet government early in Yuri Andropov's tenure as party chief, is a symbol of the evolving relationship. So, too, is the fact that the ''brief, working trip'' by Mr. Aliyev - the most senior known Soviet visitor here since Israel's invasion of Lebanon in 1982 - will be a matter of public record, with accompanying fanfare. As sources here acknowledge there has been a tendency in the past not to announce various high-level Soviet visits here.
The increasingly shared Soviet-Syrian interests have roots in shared Mideast setbacks. They include Mr. Sadat's steering of Egypt, the Arab world's most populous and powerful state, from a decidedly pro-Soviet ''nonalignment'' toward alliance with the United States and separate peace with Israel. And they include Israel's swift military thrust into Lebanon, an invasion that destroyed huge chunks of Syrian armament and left an embarrassed Moscow on the sideline.
Generally, both the Soviets and the Syrians saw themselves threatened with isolation in the Mideast - strategically, and in the Arab-Israeli diplomatic arena. Both states saw the introduction of US Marines into post-invasion Lebanon as a bid to further this isolation, and impose a new Lebanese order to the liking of an Israel closely aligned with, and actively supported by, the Americans.
''We agree on the evaluation presented by our friends in Syria on the aggressive nature of the actions of imperialism and its agents against the Arab people in general and Syria and Lebanon in particular,'' the Soviet Communist Party stated in a response two weeks ago to a Syrian policy vote.
From the start, both Moscow and Damascus had their own geopolitical reasons to hope fervently that Pax Americana in Lebanon would fail.
Now, in the view of Western and Arab diplomats here, that hope has been very nearly fulfilled.
And both partners in the Soviet-Syrian alliance, the diplomats feel, see in the pullback of the marines to naval vessels off Lebanon's coast a potential opening for a challenge to US domination of the Mideast scene over the past decade.
For the Syrians, this hope has been made flesh in a flow of visits by various Lebanese politicians to Damascus in recent days - including, Thursday, an envoy from beleaguered Lebanese President Amin Gemayel.
For the Soviets, the prospect is of an important say in moves by the European members of Beirut's multinational peacekeeping force to pull out in favor of a United Nations contingent. Moscow, as a permanent member of the Security Council , could veto any such arrangement.
The consensus among diplomats here is that the Soviets will likely want at least two conditions met before supporting a UN move. The first is a complete pullout of the marines, not just their redeployment offshore. The second is a chance to proclaim, ''We told you so,'' to the West and the world at the UN, and the chance publicly to be seen as playing an active role in a UN bailout of the multinational force.
Tension, still and all, remains built into the Soviet-Syrian alliance - even should it not surface during Mr. Aliyev's visit.
The Soviets remain keenly aware that - given the huge scale of US aid to Israel, and the program's growing significance in light of the Israelis' current economic crisis - any Syrian bid for a negotiated settlement with Israel will necessarily afford Washington the key superpower role.
Of more immediate relevance at a time when guns outweigh diplomacy in the Mideast is the danger of unmanageable military confrontation between Syria and Israel.
The Soviets are aware that Syria is their sole surviving front-line ally in the Arab world after a decade of US dominance in the Mideast. Since late 1982, they have dispatched additional tanks, missiles, sophisticated command and control apparatus, and some 7,000 military experts and advisers to Syria. Soviet personnel are understood to be manning antiaircraft missile sites. There can be no doubt that in case of a direct Israeli threat to Syria - as opposed to Syrian forces in Lebanon - Moscow would actively participate in a Syrian counterthrust.
Last September a Kuwaiti newspaper, though viewed by diplomats as often more lively than accurate, said Moscow had ''plans to transport 52,000 Soviet troops into Syria inside 12 hours'' in case of Israeli attack.
In November, a respected Arabic-language journal in London quoted Arab diplomats as saying Moscow was weighing with Syria the possibility of a Soviet naval base in the Syrian port of Tartuf ''to observe the movements of the American Sixth Fleet'' off the Lebanese coast. The report has not been independently confirmed, however.
Still, it is no accident that the Soviets' intermittent statements of support for Syria have pointedly avoided the kind of precise, or carte blanche, reference to military commitment likely to be seen as encouraging such a showdown.
As it happens, Damascus, too, has at least so far been steering clear of risk-taking on the Syrian-Israeli front despite the Soviet-supported military buildup of the past year.
Even Israeli analysts, not in the habit of saying nice things about Syrians or Soviets, speak of Moscow as a ''moderating'' influence in the Syrian-Israeli rivalry.
And they tend to view Damascus' rearmament as an essentially defensive one reflecting concern over the skewed Israeli-Arab balance of power after Egypt's peace and the Lebanon war.