Is the Syrian-Israeli war scare for real? Not if you accept the assumption that both parties know what they are doing, that both are capable of calibrating their intense war of nerves so as to avoid stumbling into open hostilities.

But suppose, as a number of observers suspect, that Syria's military and intelligence services are caught up in a power struggle over who gets to succeed Hafez Assad if his uncertain health fails. Suppose that Assad is not in firm control. Suppose, further, that the Israelis (a) don't accept that proposition and insist on holding Assad accountable for any hostile act to which a Syrian connection can be traced, or (b) do accept the ''loose cannon'' theory of the case and see it as all the more reason for a preemptive strike.

Granted, that's a lot of iffy propositions. But you can go very wrong in the Middle East if you ignore the "ifs" and go by the book. Conventional wisdom has it that Syria is a pliant Soviet client; Assad is said to be a careful, cunning dictator; any terrorist act is presumed to be "state supported."

Maybe so, but consider this big if: What would have been the consequences if Israel's exemplary security techniques hadn't turned up the bomb on its way onto an El Al 747 at Heathrow last month? The aircraft could have exploded in the air en route to Tel Aviv, with the loss of more than 340 lives, including a great many Israelis. British and Israeli intelligence would have come up with the same evidence of Syrian complicity that the United States, Britain and Israel now all seem to accept.

The punishment Israel would then have felt obliged to inflict would have been more than enough to open up a full-scale war with Syria. Only diplomatic intervention of the two superpowers, in the interest of avoiding a chilling confrontation, could be counted on by the Syrians to save them from a bloody defeat. The denouement of the 1973 Yom Kippur war between Israel and Egypt (with the United States at one point at a high state of nuclear alert) reminds us that this is a chancy business.

So, you don't have to be soft on Assad to ask what would be in it for him to start down that road.

That is precisely what Jordan's King Hussein wanted to know when he met recently with Assad in Amman. Hussein had reason to dislike and distrust Assad, having long lived under Syria's guns and as a target of Syria's expansionism. But Hussein also has more reason than most to know that Assad is crazy like a fox: He may be ruthless, but he is not stupid. And so Hussein engaged his old adversary, I'm told, in an exchange that sheds light on the question of whether Syria and Israel are headed -- perhaps unknowingly -- toward war.

Hussein began with a warning: Assad was playing a dangerous game; Israeli and Western European intelligence services were intensifying their counterterrorist efforts; the swift uncovering of a Syrian connection to the El Al incident was proof of their skills. Hussein could not believe that Assad would take such a risk. Was it possible, he wanted to know, that the bombing attempt had been undertaken without Assad's knowledge?

By one account, Hussein pressed the question three times, and each time there was a weighty pause before Assad repeated his denial of Syrian involvement. Quite the contrary, Assad insisted; Nezar Hindawi, the Jordanian arrested by the British and charged with planting the bomb in the baggage of his Irish girlfriend, had volunteered his services to Syrian intelligence and been rejected. He was rejected again, Assad's story went, when, having acted on his own, he asked the Syrian Embassy in London to help him get out of the country.

It is not clear whether Hussein was more impressed by Assad's version of events or by the pregnant pauses when he explored whether Assad was in full command. But Hussein is not alone in his suspicions. Zeev Schiff, military editor of the Israeli newpaper, Haaretz, raised that same question in an article about the El Al incident. One possibility, he wrote, is that the Syrian government ''is ready to absorb an Israeli attack'' and may even want a war. But there is ''the second posssibilty . . . that the Damascus regime has lost control over its various intelligence arms'' due to an internal power struggle.

There is even evidence the Soviets are worried that the Israeli-Syrian exercise in brinkmanship could spin out of control. One report has it that the Soviets recently raised the question of some kind of joint U.S.-Soviet attempt at preventive diplomacy. But the Reagan administration, according to this report, is going by the book: Any joint peace-keeping efforts with the Soviets in the Middle East would open the door to a larger Soviet presence. So the answer went back to Moscow that the Soviets should restrain their client, Syria, and let the United States worry about Israel.

That makes sense if the Soviets can dictate to Assad, and if Assad is calling the shots. It is the uncertainty surrounding both those ifs that makes the Israeli-Syrian war scare real.