The conventional wisdom has it that the new realities of the Middle East present Washington with unparalleled opportunities to achieve a comprehensive peace in the region.

At first glance, the evidence seems compelling. Soviet influence and credibility in the Arab world probably have never been lower. The world oil glut has reduced potential Arab oil power to its lowest ebb in almost a decade. The PLO has been driven out of Lebanon, and Israel has more secure borders than at any time since the creation of the Jewish state. The United States has the diplomatic momentum growing out of the Lebanon crisis.

But despite this conspiracy of optimism, the reality is that the Reagan peace plan is unlikely ever to get out of the starting gate. The simple reason: Begin and Defense Minister Ariel Sharon have their own "peace plan" -- and their plan has already been largely implemented.

The world according to Begin and Sharon looks pretty rosy. Israel has achieved peace -- or at least a state of nonbelligerence -- with all of its neighbors. It has a peace treaty and normalized relations -- albeit seriously frayed because of Lebanon -- with Egypt. It has de facto open borders and robust trade relations with a Christian-dominated Lebanon. And through its awsome military prowess, it has for all practical purposes intimidated into submission its two remaining neighbors, Syria and Jordan.

Now the world's fourth largest military power (according to the presitigious Institute of International Strategic Studies in London), Israel has such vast military superiority that no combination of Arab forces could contemplate another Mideast war for at least a decade. And, as underscored by Arab impotence during the invasion of Lebanon, the Arab world is weaker and more divided than at any time since the death of Egyptian President Nasser.

As for the occupied West Bank and Gaza strip -- the core of the Palestinian problem -- Begin is closer than ever here to achieving his long-standing dream of "Eretz Yisrael" (Israel's Biblical borders).

The Begin solution to the West Bank question is, like much of his peace plan, based on sheer military force. It is a strategy of attrition, seeking to wear down Palestinian opposition while pouring Jewish settlers into the area.

Already there are more than 30,000 settlers on 103 settlements. As announced by Israeli officials, moreover, there are to be 20 more settlements on the West Bank over the next year, part of a plan to boost the Jewish population to 100,000 by 1986 and to more than a million by the beginning of the next century.

It is useful to recall the trauma in Israel last April when the Begin government forced Israeli settlers in the Sinai town of Yamit to leave. Looking at the West Bank, one can multiply the difficulty by a hundred to fathom how much of a problem Israeli withdrawal from the West Bank -- as called for by the Reagan plan -- would be.

But the West Bank still poses the most immediate and profound problems for Begin.

He is unlikely to push for formal annexation of the occupied territories, because to do so would pose deep philosophical and religious problems for Israel. Formal annexation would mean that 1.2 million Arabs would become Israeli citizens. Added to some 800,000 Arabs already in Israel proper, that would mean 2 million Arabs in a state of 4 million Jews. This would profoundly alter the character of the Jewish state.

So Begin, a lawyer by profession, is aware of the need for a legal fig leaf for his de facto absorption of the West Bank into Israel.

This dilemma is hidden in the frequently used shorthand of "Autonomy for the people, not the land," used by Begin in reference to the West Bank. But the sort of autonomy Begin has in mind is radically different from that conceived of by Egypt and the United States, his Camp David partners.

Failing the legitimacy that a negotiated autonomy through the Camp David peace process would bring, however, Begin could simply impose his own autonomy plan on the West Bank. He has already moved in this direction, replacing elected Arab officials in the occupied territories with hand-picked "leaders" regarded even by many prominent Israelis as merely quislings.

Like the 800-pound gorilla, the Begin government can sit anywhere it wants on the West Bank. Who is going to do anything about it?

This is the reality that President Reagan will face when he meets with Begin on Friday. Any serious effort to move ahead on an American-brokered, comprehensive peace plan must start with this assessment. Otherwise it is either naive or disingenious.

This doesn't mean that the Reagan plan is not well conceived, reasonable and balanced. It is. Nor does it mean that "moderate" Arabs such as Jordan's King Hussein would not like to get on board -- even if for reasons of desperation rather than good faith.

But it does mean that unless the administration is prepared to go to the mat with Israel in a manner unseen since President Eisenhower's tough action in the 1956 Suez crisis, the Reagan plan is a dead letter.

Neither King Hussein nor any other Arab leader will consider signing on without solid assurances from Washington that at least in broad terms, Israel is prepared to trade territory for peace. This is the essence of the Reagan plan, and it stands in direct contradiction to Begin's own "peace plan." Since Begin has accomplished most of his goals already, he has little incentive to make concessions.

The ebb of Soviet influence and of Arab oil power, combined with a status quo created and enforced by the Israeli military, might have reduced the potential explosiveness of the region. But this, unfortunately, makes it even less likely that the Reagan administration will risk taking bold moves to press Israel, and more likely that the Middle East will muddle along as usual.