There is a missing link in the understanding Americans have of the circumstances that led to the disruption of peace negotiations between Egypt and Israel. And that "link" is an eye-opening statement by Egypt's president, Anwar Sadat, that, although highly significant, has never, to the best of my knowledge, been published in the United States.

As a result, the impression in the United States is that Israel's prime minister, Menachem Begin, is the villain responisble for the breakdown of peace efforts, and Sadat is seen as the innocent, long-suffering victim of Begin's alleged "intransigence."

Now that Vice President Mondale has arranged for the Egyptian and Israeli foreign ministers to meet under U.S. auspices in London on July 18, it is time for Americans to get a clearer idea of what derailed negotiations last January.

The unpublished story is far different from the accepted one. It springs from an exclusive interview given by Sadat to his favorite personal mouthpiece (the Cairo weekly October) on Jan. 1.

It is essential to keep that date in mind to understand the contradictions in Sadat's shifting positions. Sadat now says Begin stonewalled him after his historic mission to Jerusalem last November. He accuses the Israeli leader of being inflexible and hawkish as well as insensitive to Egyptian problems. Sadat says his Jerusalem initiative failed to evoke either concessions or compromise proposals from Begin.

The Israeli prime minister, however, has a radically different story. Following Sadat's trip to Jerusalem, Begin in December made a return visit to Egypt, conferring with Sadat in Ismailia.

According to Begin, everything went serenely. He found Sadat open-minded about Israel's general proposals and quite ready to leave specifics to be negotiated by joint political and military committees created at Ismailia. He returned to Israel feeling that a "warm" relationship had been established, and that a peace agreement was imminent.

What is the truth about Ismailia? Was Begin cooperative, or, if we are to believe Sadat, was he negative and unbending? Let us now hear from Sadat as he personally described the Ismailia meeting soon after it took place:

"It had been agreed that we should deal with general principles in Ismailia," Sadat told October. This in itself, he said, was "a very important achievement. However, what happened was something more important and greater than this. Begin came and brought a complete plan on withdrawal from the occupied territories."

It was, Sadat said, "the first time since the establishment of Israel that the Jews presented something specific . . . This time they went further than one could imagine . . . they came to argue with us. I would state my views and they would state theirs. This in itself was a positive step. We both agreed to discuss everything. Begin had declared that everything was negotiable."

Sadat frankly said that he had previously regarded Begin as the "hawk of hawks." But suddenly, after the trip to Jerusalem, Sadat said he found that Begin "had changed his opinion and wants peace, not war. He wants discussions . . . and we would leave the task of discussing details to the ministers" of the joint committees created to carry on the negotiations after Ismailia.

"Therefore," Sadat observed, "it is not true that we did not agree on anything, or that I did not obtain any specific things." The Egyptian president said that when he saw Begin in Jerusalem, he realized "that he was a man with whom understanding can be reached." He said he had learned from President Nicolae Ceausesau of Romania "that Begin is a man who truly wants peace . . ."

That was on Jan. 1, but four days later Egypt's government-controlled press began a campaign to undermine the negotiating committees established at Ismailia by Sadat and Begin. Sadat was quoted as saying that the negotiations, scheduled to start on Jan. 14, were doomed in advance because of Begin's supposed intransigence.

The joint political committee, however, after only one session in Jerusalem, made such phenomenal progress toward a peace agreement that the diplomatic world was stunned when Sadat, without warning, cancelled the meetings and abruptly called home his negotiators.

What prompted this seemingly inexplicable change of heart? What happened between Jan. 1 and Jan. 5? In diplomatic circles, interest centers on Jan. 4 when President Carter, after a trip to Saudi Arabia, held an unplanned meeting with Sadat at Aswan.

We will probably never know for certain what took place, but the present consensus is that Sadat, rightly or wrongly, got the impression that Carter, through pressure on Israel, might be able to get him a better deal than he had been prepared to settle for at Ismailia.