Anwar Sadat's assassination has brought home to the United States that the hopes Americans pinned on the peace process he initiated are shattered. It is even questionable whether Israel's withdrawal from the rest of the Sinai, as called for in the Egyptian-Israeli peace treaty, can be taken for granted.

The treaty stipulates that "full normalization" of relations between the two countries should precede total withdrawal. But withdrawal is a phased military operation due to be completed in April 1982. Normalization is a process that does not depend exclusively on commitments by governments; it is something that must mature and for which no deadline can be fixed.

Sadat scrupulously abided by the letter of the treaty. But the Israelis cannot have failed to notice that their presence in Egypt has come up against manifestations of hostility from an ever growing opposition, including forces that cannot be accused of fanaticism and anti-Semitism.

How can any Egyptian condone as "normal" the Israeli government's claim that Arab Palestine does not exist, its decision to make Arab Jerusalem Israel's capital, or Israeli Prime Minister Menachem Begin's assertion of a right to strike preemptively at any Arab state?

No doubt Begin feared that Israel would eventually be obliged to withdraw before normalization could ensure the irreversibility of peace. He may even have accused Sadat of deliberately adopting an equivocal stand toward his internal opposition, in preparation for his re-entry into the Arab world once Sinai was restored.

Sadat's clampdown on all opposition forces in Egypt, including moderate and secular critics, came only a few days after his meeting with Begin in Alexandria and more than two months after the Zaweyat-El-Hamra incident involving violence between Moslems and Coptic Christians. It may be that Begin threatened not to withdraw if forces opposed to normalization were not dealt with decisively. Because of the clampdown, Sadat was assassinated one month later.

Because Hosni Mubarak is not Sadat, Israel will be still more demanding on the issue of normalization as a condition of evacuation and as a means to test the new president. From the other flank, the Arabs are expected to offer Mubarak the opportunity to bring Egypt back to the Arab fold. With the authority of the state challenged and opposition developing into civil disobedience and outright acts of insurrection, the temptation for him to come to terms with the Arabs will become irresistible.

The temptation will also be strong to identify a villain so that Egypt's new power buildup will be less subjected to the opposing pulls of Israel and the Arabs. This would be Libya's Qaddafi. Such a way out of the impasse fits the Reagan concept of a "strategic consensus" against the Soviet threat that Qaddafi is assumed to personify.

This will, however, identify U.S. policy in the Middle East with war, not peace. It must also be asked whether a war with Libya could not backfire, with the spread of Islamic fundamentalism in the ranks of the Egyptian army. A military operation against Qaddafi--even a sucessful one-- would make him an Arab hero.

The breakdown, with Sadat's death, of the restructuring of the Middle East that he sought has encouraged even former American presidents to call for a dramatic re-evaluation of U.S. policy. Both Jimmy Carter and Gerald Ford have gone so far as to advocate PLO participation in the peace process. This is what Sadat himself recommended during his last visit to Washington, only to be totally rebuffed by Ronald Reagan.

But for Begin, the PLO, like AWACS, is a code word to be read as adopting the Arab stand against Israel's security and survival.

Experience has shown that separate settlements can eventually backfire and make a comprehensive peace more remote. An appreciation of the need for a comprehensive approach must be the point of departure for a total reappraisal of the Middle East equation.