THE EVACUATION of Beirut by the PLO proeeds apace -- the single smoothest example of international cooperation in the Middle East in memory. It begs belief, however, that subsequent phases will go so well. The parties have a brief respite. Then come new tests.

The most immediate is the fate of the foreign armies remaining in Lebanon. The PLO's forces in the countryside north of Beirut would seem to be living on borrowed time -- outnumbered, outflanked, abandoned by their leadership. The Syrian occupation force left over from the 1975-76 Lebanese civil war is another story. Lacking among other things effective air defense and meaningful Arab or Soviet support, it is extremely vulnerable. Israel's defense minister has pointedly noted that Israeli artillery now sits within range of Damascus. Israel's determination to remove the Syrian troops, and to humiliate President Hafez Assad, is evident. Mr. Assad, beset on other fronts, does not appear to be in a very flexible position or mood.

There may be another Israeli-Syrian explosion. American diplomacy needs to be thrown into the breach to avert one. It is urgent to set up a framework in which, in the context of restoring the Lebanese government's authority, all foreign forces quit Lebanon on a coordinated schedule. If the Beirut evacuation can occur, something similar can occur in the country as a whole. Israel declares its army won't stay "one more day" than Syria's. It should be held to its word; there can be, for instance, no accepting of any future Israeli insistence on obtaining certain conditions from Lebanon, like Litani water rights, in pre-withdrawal negotiations over a peace treaty. It is a vital interest of both Israel and Syria that the other not threaten it from positions in Lebanon. What is needed is an extension of the disengagement principle that the two countries acted on together, with American assistance, in the mid- 1970s.

Then there is the Palestinian question. It is painfully obvious that Israel means to use its Lebanon momentum to solve it in its fashion. Foreign Minister Yitzhak Shamir outlined the plan on the op-ed page last Thursday: immediately re-open and quickly complete talks on autonomy with available West Bankers who are neither associated with the PLO nor "supporters of the PLO ideology," and then negotiate with them -- conceivably only with them -- the "final status" of the territory. You could call this an "internal settlement," one imposed by the occupying power and almost certain to fail to get international backing. President Hosni Mubarak has already indicated, on the op-ed page last Sunday, that Egypt will boycott any such process.

Let us pay close attention here: Israel has a fair point in demanding that any prospective Palestinian partner accept it and negotiate with it. The PLO or any successor must ultimately meet this test, or stay unrequited. Certainly, it is ridiculous of the PLO and those sympathetic to it to endorse a no- acceptance standard enforced by violence and intimidation and to write off all who reject it as "quislings." Yet the Israeli plan to deal with a minority of the dependent West Bankers without a proven political base is a formula for ensuring permanent Israeli breach with its Arab neighbors -- with all of the implications for the American regional position that entails. It makes a mockery of the purposes the United States accepted at Camp David. The administration is duty bound to oppose it.

But how? The Egyptians feel they can best steer Israel back to the true Camp David path (and serve their other purposes) by boycotting the intended autonomy talks and by putting a frost on their other dealings with Israel. The United States will have to work out its own appropriate forms of persuasion. Waiting out Menachem Begin has only a superficial appeal: it tells the Israelis to elect another Begin. The idea of inviting a great confrontation with Israel has its champions but is irresponsible for a great power. Sniping and nipping at Israel-a policy of harassment for Arab-oriented show_comes easily to many in Washington but is undignified and can accomplish nothing in the absence of a clear master plan.

In the end, there is no substitute for a firm but careful approach in which the United States makes two propositions totally clear: its deep concern for Israel and its determination to serve its own broad interests as it sees them. We cannot believe that responsible officials or citizens in any country would wish to force the United States to a choice.