IN THE RELIEF at the end of the siege of Beirut and the excitement over the Reagan peace plan, people have tended to forget the explosive situation that still exists in Lebanon between Israel and Syria. Early in what might be called Phase I of the Lebanon war, Israel's air force entirely stripped the 30,000-strong Syrian force in Lebanon of its missile defense and air cover. Now Israel sits with a large and vastly superior army within artillery range of Damascus and demands that the Syrians retire behind their own border. To enforce that demand, and to retaliate for raids on its lines by PLO units sheltered by the Syrians, Israel conducted a day-long series of raids yesterday up and down the Syrian-occupied Bekaa valley. A nasty Phase II could break out at any moment.

The action in the Bekaa valley recalls the Beirut pattern in which the Arab side hung on and the Israelis piled on the pressure in order to make a diplomatic denouement possible. But there is a major difference. In Beirut, the Israelis could at least make a case that a siege was necessary to rout a PLO presence that endangered Israel. By no valid, commonly accepted criterion can the Syrian presence -- weak, vulnerable, remote from Israel proper, almost entirely defensive in its military particulars -- be said to endanger Israel. On the contrary, the Israeli presence, looming over the approaches to Damascus, is a plain threat to the vital interests of Syria. Israel is exploiting its penetration of Lebanon to accomplish an add-on strategic objective -- and perhaps also to humiliate President Assad. With American weapons.

The American fire brigade was busy at work yesterday to dampen the immediate crisis. The pressing need, however, is for the departure of all foreign forces, Israel's, Syria's and the PLO's -- each in its fashion a tormentor of poor Lebanon. Only if they leave can the risk of further collisions be reduced. Only then can Lebanon breathe.

Since Israel has now destroyed the PLO forces whose disruptions in Lebanon first stirred the Arab League to send in a Syrian "peace force," the recent Arab summit at Fez found it possible to agree in principle on Syrian withdrawal, as long as Israel also withdraws. But the essential details were fudged. Meanwhile, Israelis suggest that Syrian withdrawal may not be enough to ensure Israeli withdrawal: certain security arrangements in Lebanon may be required first, for instance, or a Lebanese-Israeli peace treaty. Each side, in other words, is nourishing the suspicions of the other. There may be very little time left.