More than a third of the Soviet military advisers in Syria have been withdrawn in the last six months, according to western sources here. These include an air defense unit that was the only potential Soviet combat force in the region, the sources said.

Senior Syrian military and civilian officials would not comment on the motives for the withdrawal or on any specific aspect of their armed forces.

But the decline in the number of Soviet advisers from a high of 6,000 to 4,000 or less is viewed among diplomats here as an indication of the careful control Syrian President Hafez Assad maintains over his country's relations with Moscow.

"The Soviets have basically only one major foothold in Syria and that is the arms relationship," one diplomat said.

Another western envoy suggested that with the exception of its military hardware, "Syria has nothing in common with Soviets."

Sources here said the main body of the Soviet air defense unit, involving troops manning the SA5 surface-to-air missile batteries, was pulled out in October, and smaller groups of advisers have been moved out regularly since then. Although some Soviet "fire control" over the SAM systems may be maintained, they said, the Syrians are thought to run them mostly on their own now.

According to one usually well-informed source, the number of Soviet advisers may be as low as 2,000 to 3,000, with most of them scattered around in training, logistical and staff positions.

The Reagan administration has portrayed its strategic alliance with Israel partly in terms of Washington's overall desire "to contain communist aggression" in the Middle East, and Syria is cited as the spearhead of the Soviet threat here.

But experienced western diplomats in Damascus, without trying to defend Assad's policies toward Israel, point out that his government often pursues its own course with little attention to Moscow's wishes.

The most frequently cited example was the Syrian decision in 1976 to commit troops to Lebanon in an effort to stabilize the situation there. The Soviets opposed the move and sent Foreign Minister Andrei Gromyko here to make the point, according to diplomats. The Syrians went across the border anyway, not bothering to inform Gromyko until the action was an accomplished fact.

Soviet arms supplies to Syria dropped dramatically after that, not picking up again until 1978, western diplomats said. But after 1982, when Israel invaded Lebanon, the scale of Soviet arms supplies rose dramatically.

Direct engagements with Israel's U.S.-backed forces cost the Syrians 98 aircraft, including helicopters, MiG21s and MiG23s, according to western sources. Attempting to move along the one narrow highway leading into the Bekaa Valley, an entire Syrian armored brigade was destroyed.

Since then, Assad has sought Soviet help to replace what was lost and to begin building a force that could claim a "strategic balance" with Israel.

With Syria's traditional military allies against Israel now out of the picture -- Egypt having signed a peace treaty and Jordan apparently leaning in that direction -- the Syrians concluded that "they can count on no one, so they must count on themselves," one diplomat said.

Speaking in general terms, Foreign Minister Farouk Sharaa said Syria's goal is "to be strong enough to defend ourselves against any future Israeli aggression" and to be able to negotiate eventually without "the Israelis being able to dictate their conditions to us."

To pursue such a policy in the face of Washington's firm commitment to Israel, Syrian officials said, they naturally turned to Moscow.

The goal of rebuilding the Syrian armed forces has been met now, according to western sources. Syria is reported to have more than 500 combat aircraft, including a few sophisticated MiG25s. It has roughly 3,500 tanks and more than 2,000 artillery pieces. Its armed forces total about 400,000 men, of whom 40,000 are in the Air Force and 60,000 in air defense units.

The Soviets have also supplied SS21 surface-to-surface missiles, which, one military source here noted, are tactical weapons in Europe but have strategic value here where distances are small and Israel's major cities so close.

In numbers of men and materiel, the Syrian armed forces can claim to be approaching Israel. But the aim of "strategic balance" is still far away, these sources said, because of Israel's vast technological advantages.

"There are certain areas where the Syrians have increased their capabilities," said one source. "But the Israelis are not standing still, either." While Syria relies on the Soviet Union for arms, Israel, this year, is getting $1.4 billion in military aid from the United States and is seeking more next year.

A major element in the Soviet military buildup here in the last two years was the introduction of SA5 antiaircraft missiles and a special air defense unit of about 1,500 Soviets to run them, according to western sources.

"The Soviets came in with the SAM5 systems in 1982 after mutual recriminations," one source said. Moscow blamed the Syrians for their humiliation at the hands of the Israelis. Syria blamed the inferiority of Moscow's equipment. "The Soviets had to show solidarity some way, and this was done with the air defense unit, which was the only Soviet combat unit here," the source said.

But even the SAMs are regarded as less than impressive and their Soviet crews stayed well back from any potentially vulnerable positions.

"If the SAM5 hadn't been put here," a European diplomat said, "it would have been on the scrap heap. It doesn't cost the Russians anything. It is a 25-year-old system."

Many Syrians and western diplomats note that, despite policies of strong government control over the industrial economy here, the cultural inclination of Syria is much more toward the West, particularly Western Europe, than toward the Soviet Bloc. From the end of World War I until regaining independence in 1946, Syria was administered by France under a League of Nations mandate.

It strikes some western observers as curious that the Soviet military presence here is being reduced now, as tensions are rising in Lebanon and the factional fighting between Syrian-backed forces and Israeli-supported units is increasing.

But others note that the Syrians, well aware of their continued military shortcomings, are extremely careful to avoid any direct confrontation with the Israelis at this point, do not expect to have one now, and probably would not expect much from the Soviets if they did.