For a capital which could be at war any day now, Damascus today seemed oddly serene -- reflecting President Hafez Assad's apparent conviction that Syria has much to gain and little to lose from a limited conflict with Israel in Lebanon.

Convinced that Syria is the injured party in the dispute over its placement of Soviet-built antiaircraft missiles in Lebanon's Bekaa Valley, Assad gives every sign of welcoming a chance to stand up and fight, according to Syrian officials and diplomats interviewed here.

For those unacquainted with the often baffling intricacies of Middle East thinking, official Syrian logic may seem skewed. But Assad, it should be recalled, has remained in power here a decade -- longer than any modern Syrian ruler.

If Israeli aircraft wipe out the Soviet-built ground-to-air missiles, Assad is thought to believe, Syria at least will have saved its honor by refusing to knuckle under to demands that are considered humiliating here. And, as the late Egyptian President Gamal Abdel Nasser demonstrated in the 1967 Arab-Israeli war, a clever Arab leader can turn military defeat into political victory.

If the now-extensive Syrian air defense system, backed by a considerable ground force protective screen, were able to down a single Israeli aircraft, Assad could claim a victory of sorts. Since the 1973 war, Syria has lost at least 15 planes to Israeli pilots without bagging a single Israeli aircraft -- a demonstration of the superiority of American over Soviet technology that has rankled Moscow as well as Damascus.

It was the Israeli air superiority, in fact, that led Syria to install the missiles in Lebanon following the Israelis' downing of two Syrian helicopters two weeks ago. The missiles were seen as the only weapons system providing even a gambler's chance of facing down Israel.

In Syrian eyes, an unholy alliance of neighboring Israel, Jordan and Iraq -- with the United States pulling the invisible strings offstage -- was plotting Assad's overthrow. If Syria backed down in the Bekaa Valley, then Assad would have suffered a humiliating -- and perhaps eventually fatal -- diplomatic setback.

But if Assad wins on his own terms, he will have scored strategically while rallying round him Arab leaders who now have isolated him, Syrian officials reason.

President Anwar Sadat of Egypt, whom Assad despises, might well be forced into gestures and acts that could provide Israel with the very excuse it wants to refuse to withdraw next spring from the final slice of the Sinai Peninsula it occupies. Syria would have also demonstrated that the friendship and cooperation treaty it signed with the Soviets last October could produce results.

All these considerations justify the tough Syrian stand, but officials admit they all also depend on one perhaps fatally risky assumption: that once begun, the conflict will remain localized in Lebanon, both in the Bekaa Valley and in the south where Syrians and Israelis have a common objective in cutting the Palestinian guerrillas down to manageable proportions.

The problem is that Israel may be tempted to go beyond Lebanon's sideshow theater of operations and strike directly at Syria.

The lingering fear here is that Israel may opt for an all-out grand slam against Syria on the theory that casualties from such a wider conflict would be only incrementally greater than those involved in a Bekaa Valley operation.

The Syrian infantry troops in battalion strength that were recently sent to defend the Bekaa Valley missile sites were drawn, according to reliable sources, from the supersensitive Golan Heights. That might be a green light to Israel for a broader operation -- and the kind of error in judgement which has cost Syria so dearly in the past.