Just before returning to Israel, Prime Minister Menachem Begin met at breakfast with some old journalistic friends, during which he spoke privately of Anwar Sadat, the Egyptian president, almost as effusively as he did of him publicly following the Camp David agreements.

The prime minister exuded friendship and even trust for his old antagonist. He believes his feelings are reciprocated, and that Sadat's apparent readiness for reconciliation is as genuine as his own. Begin is confident that this time there will not be another derailment of their joint peace plans.

It can only be hoped that he is right, but similar euphoria was inspired by Sadat's historic trip to Jerusalem last November, and expectations were raised even higher in the wake of Begin's return call on Sadat at Ismailia in December. Yet it all came to naught when the Egyptian leader later had a sudden and still puzzling change of heart.

It is well to keep in mind that the terms of the agreements reached at Camp David, despite their artful drafting, are in substance little different than the broad proposals that Begin took with him to the Ismailia conference, where he and Sadat appeared to get on so amicably.

After that meeting, Begin says, "we parted in warm friendship," with the Israeli thinking he had heard "encouraging expressions" from the Egyptian president, who readily agreed to the establishment of joint committees to carry on the negotiations to a conclusion.

Sadat himself said that Begin "came [to Ismailia] and brought a complete plan on withdrawal from occupied territories." It was, he added, "the first time since the establishment of Israel that the Jews presented something specific. . . . This time they went further than one could imagine. . . . We both agreed to discuss everything. Begin declared everything was negotiable."

The Egyptian leader frankly said that he had previously regarded Begin as the "hawk of hawks." After his trip to Jerusalem, Sadat said, he found that the Israeli prime minister "had changed his opinion, and wants peace, not war," Moreover, he observed, "it is not true that we did not agree on anything, or that I did not obtain any specific thing."

Yet, a few days after saying all this, Sadat abruptly terminated the peace efforts, and ordered his negotiating team to return from Jerusalem, where they had been making encouraging progress. Thus began the long stalemate, which lasted unil Camp David.

Middle East experts are still trying to figure out what happened. Some think Sadat was intimidated by the violent objections of the Arab "Rejection Front," but now there is a growing belief that a more significant factor was pressure by the Egyptian foreign ministry, long packed with hard-line, die-hard anti-Israel officials.

When Sadat first proposed to go to Jerusalem, the then foreign minister, Ismail Fahmi, resigned in protest. And now, at the climax of the Camp David negotiations, Fahmi's successor has also quit as a result of Sadat's latest rapprochement with Israel.

How will the Egyptian president weather the heat this time - not only from other Arabs and the Palestine Liberation Organization, but from his own intrenched bureaucracy? Begin's private guess is that Sadat won't fold.

The first time around, Sadat's foreign advisers, along with others, persuaded him that if he held out there was a good chance that President Carter could get him a full-fledged "comprehensive" agreement by bringing pressure on Begin. Events have disabused Sadat of that fantasy, so he has now largely settled for what he has now largely settled for what he could have had all along - a separate peace with Israel.

He need not feel defensive about this, for he has obtained for his own country every concession it could legitimate have sought. Not many other defeated nations have been able to win back all their territorial losses through peaceful negotiations with the victor.

Before leaving Washington, Sadat also met with an invited group of journalists, and emphasized his determination to carry on his peace effort alone if necessary. "I shall survive," he said, "because I have 40 million Egyptians behind me."

That wasn't much of an exaggeration, for the average Egyptian has little or no stake in pan-Arabism of resolution of the Palestine question. After four impoverishing wars, the Egyptian people want and need peace above all else, and that's what Sadat is now in a position to deliver. If he does, he could well end up in an enviable position both at home and abroad.