THE LESSON of the last fortnight in the Middle East in this: the region is in a recognizably new phase that requires a review of positions the United States has not examined in years.

This phase began with the re-election of Menachem Begin. Thus fortified, he set out to destroy the Palestinian military pressence in southern Lebanon. He also appears to have had it in mind to reduce the Palestinians as an independent negotiating factor at least over the next four years. The PLO, responded with the first rocket attacks in more than a year that actually killed some Israeli civilians. In return, the Begin government killed hundreds and wounded hundreds more Lebanese and Palestinian civilians, openly stating its intent to turn civilians against the guerrillas.

It is the totality of this situation, entailing not just the mutual targeting of civilians but also the torpedoing of already-gloomy negotiating prospects and the resulting damage to American interests, that the United States must now address.

Israel has a right to defend itself. But it does not have a right to label any excess of preemption or retaliation self-defense and then expect others to accept this reading. And the fact is that Mr. Begin has been dissipating Israel's most precious asset in the United States, its claim to a moral edge.

Some people suggest that the attacks are Mr. Begin's answer to the post-Iraq American plea to show restraint in the use of American-supplied weapons. And from this they conclude that Mr. Reagan should keep in effect, or extend, his suspension of deliveries of F16s. A case can be made that the United States needs to make such a gesture to salvage standing with friendly Arab states. But the jiggling of arms deliveries is a limited and in most respects bad idea. It runs against the fundamental American purpose to keep Israel strong and secure, the better to enjoy its national life -- and to be able to make political accommodations. Precisely because maintaining a certain level of such aid fulfills American as well as Israeli objectives, restricting aid is unconvincing and, thus, ineffective and even humiliating: Mr. Begin and everyone else knows that the arms tap will be turned back on.

Jiggling deliveries also has this fatal disadvantage: it does not address the central political question, the fate of the Palestinians. For 33 years the United States, recognizing a special debt to a people ravaged by Hitler's Holocaust, has sustained the Jewish effort to build a homeland in part of Palestine. This has been a proud chapter for Americans. But Israel remains besieged, in a sense never more so, and the United States, Israel's only friend and patron, itself pays major costs. The reason is that the Palestinian issue is not being properly addressed.

For this, Israel and the United States are not the only responsible parties. The Arabs, including the Palestinians, share heavy -- and bloody -- blame. The Palestinians could have had their state in 1948 from the United Nations or until 1967 from Jordan. The PLO's basic charter still upholds a policy of terror. But though many peoples have lost their homeland and are ignored, the Palestinians' claim to nationhood does have a moral basis. The reality of Arab and global politics gives it political urgency.

FOR MANY years the United States followed Israel's lead in pretending there was no Palestinian question. Under Jimmy Carter, neglect yielded to an intense search for a formula for Israeli-Palestinian reconciliation, with a two-state result always the gleam in Mr. Carter's eye. But it was the mote in Mr. Begin's. The Carter quest fell short.

Although ideally a solution would be reached by negotiations. Mr. Begin's Palestinian policy early on mooted the question of whether the Palestinians could ever be drawn in to Camp David. From his extension of Jewish settlement in the West Bank through his niggardly approach to the Palestinian autonomy talks to his recent raids in Lebanon, the Israeli prime minister has ensured that Palestinian readiness for a political solution could not be fairly tested. He makes no bones about this. In his inaugural address in 1977 he disconnected Israel from a demand to be recognized by the Palestinians -- a demand creating a reciprocal obligation. Mr. Begin does not say that under certain conditions he would accept a Palestinian state. He says that under no conditions will he.

How then can it be learned whether, as some Palestinians hint and as their friends insist, the Palestinian movement is ready for a two-state coexistence solution? Here we arrive at another obstacle: the American pledge of 1975 not to deal with the PLO unless it first accepted Israel's right to exist in word and deed. Is this pledge still vital?

The premise of that pledge was the legitimacy of Palestinian nationalism. It was intended as a tactical lever to bring the PLO to accept Israel's legitimacy: then it would be the Palestinians' turn. But Mr. Begin has blocked that, too. Now the United States is locked into insisting on conditions that, even if they were met by the PLO, Mr. Begin would dismiss as meaningless and irrelevant. The American no-dealing pledge is thus being honored, although Mr. Begin's Camp David pledge to submit the "final status" of the West Bank and "the location of the boundaries" to negotiations with Palestinians is not. Yet these promises are related to one another.

Another way of answering is by reference to the practical consequences. We refer here not simply to the evident advantages to American security, political and economic interests of going with the moderate Arab flow supporting Israeli-Palestinian coexistence. In the current crisis in Lebanon, an Israeli-Palestinian cease-fire is plainly in the American interest. A cease-fire securing the northern Israeli border is just as plainly in Israel's interest. The 1975 pledge keeps the United States from talking with the PLO about a cease-fire.

A third way of answering would be to underline that the sole purpose of dealing with the PLO would be to draw it along the negotiating track laid out, and agreed to by Israel, at Camp David: not to "reverse alliances" or to abandon Israel but to secure for Israel the condition of peace it has been unable or unwilling to try to secure for itself.

Dealing with the PLO would indeed be to allow a terrorist group to shoot its way into eventual respectability. But it makes a difference that the PLO is a terrorist organization and also an authentic representative political organization, that Mr. Begin himself trod this path, and that the Israeli military is now conducting the policy it is. What makes the most difference is what the PLO would be "shooting its way into."

The PLO would not be extinguishing Israel, which has its own strength and -- still and we hope forever -- a friend in the United States. The PLO would be "shooting its way into" a process intended to lead, if all went well, to a state of its own next to Israel. It could not conceivably he allowed such a state if it did not accept the usual neighborly obligations. Given the PLO's record and makeup, heavy extra obligations would be in order.

These questions ought to be at the top of the administration's mind as it composes a Middle East policy over the next few months. It is especially important to consult with Anwar Sadat, not least because an American PLO initiative could conceivably lead Israel to break off at least temporarily the peace process it has been conducting with Egypt.

If it is true that the Israeli leader refuses to open to the Palestinians the avenue that the Camp David parties, including Israel, promised to open to them, then this changes everything. And if this is true, Mr. Begin must be told that, failing to alter this policy and to commit himself otherwise, he is driving the United States to deal directly with the PLO -- for the purpose of securing from it the concessions to Israel that the United States has always hoped the PLO would make. Far from abandoning Israel, the United States would be asking the PLO to accept publicly and unequivocally Israel's right to exist, to enter direct negotiations with Israel and to repudiate terror. The American responsibility to Israel, to Arabs and to its own interests requires no less.