The Syrian government is quietly and systematically girding for what it appears to expect will be a new military conflict with Israel. According to Syrians and Western diplomats, all the known preparations are defensive and are concentrated around the Lebanese border, where Israel has threatened to destroy surface-to-air missile batteries that Syria moved there 17 days ago.

Syria has increased its air defenses near Lebanon's Bekaa Valley, where the missiles are located, and military analysts here say Israel could lose one or two planes if it attacks the SAM sites. Furthermore, the Syrian Army has moved about 2,000 men and about 125 Soviet-made tanks to the Lebanese border.

There have been no signs here that Syria has beefed up its forces along the Golan Heights, where it borders Israel, indicating to a wide variety of diplomats here that Syria expects Israeli attacks to be confined to the Lebanese border.

There is additional speculation that Israel might couple an air attack on the SAMs in the Bekaa and along the Syrian-Lebanese border with a land attack against Palestinian bases in southern Lebanon -- where there are no Syrian troops.

A wider Israeli attack could unravel Isral's peace treaty with Egypt. "To attack the Golan raises the stakes very much," said a Western diplomat. "The risk of Soviet involvement is higher if Israel carries out attacks within Syria than if it only attacks the SAMs in the Bekaa and on the border with Lebanon."

Despite strongly worded editorials and government statements on the radio that an attack on the missile sites is expected, along with a Syrian retaliation, there is no sign of general military mobilization. Some Syrian reserves appear to have been called to active duty. There are reports circulating here that regular soldiers are going to homes of selected reservists, telling them to join their units.

Moreover, the Syrians are clearing beds for expected casualties by discharging minor cases from civilian hospitals. This was described by one Syrian as a normal civil defense precaution.

On the city streets, however, the people seem to be going about business without much concern that they are in the center of an international crisis that could lead to war.

There are no air-raid drills or sirens sounding in the city. There do not appear to be extra defense preparations in the military headquarters buildings alongside the Meridien Hotel, which is located in an area that incurred heavy Israeli bombing in the 1973 war.

During that war, Syria's SAMs had a devastating effect on Israeli air power. The Syrians claim to have downed 95 U.S.-made jets over Damascus in the first two days of the war, some with SAMs and others in dogfights with Soviet-made Migs. Independent estimates of Israeli losses are lower, with the U.S. Defense Department figure for total losses on the Egyptian as well as the Syrian front put at 120 planes.

While the SAMs have been improved, so have the Israeli aircraft. They are believed to be equipped to deflect electronically the Syrian missiles or to cause them to explode prematurely.

The distances between potential combatants are surprisingly short. The missiles in Lebanon's Bekaa Valley are less than 20 miles from the Syrian border and 35 miles from Damascus. They are also just 35 miles from Israel's borders.