Prime Minister Menachem Begin today presides over Israel in a time of great uncertainty. Relations with the United States, the one indispenable ally, rarely have been worse. Peace negotiations with Egypt remain stalled and Israel is taking most of the blame in the eyes of a fearful world - fearful lest a unique chance for peace dissolve into another cycle of Middle East violence and war.

Yet, in an interview Tuesday with The Washington Post, Begin appeared calm. He admitted there were difficulties, yes, but he remained totally convinced in the justness, indeed the righteousness of his justness, indeed the righteousness of his cause and the fairness of his peace plan, which he said has not been given a proper hearing.

Despite the pleadings of many Israelis and Americans, Begin sees no reason to adopt new tactics or to change Israel's stand in the face of adversity. Some public opinion is against him, but by no means all. The cup is half full as well as half empty.

Begin believes he is right and a Begin who believes he is right is not a man easily shaken.

The prime minister said he had no doubt that the majority of Israelis were behind him, as were many people "of good will" around the world.

"We face difficulties, we do. But why should we give up a just plan? I think we have proved we want peace. Some people were surprised at our peace proposals. I had to have a serious debate among my own supporters. This is quite painful for everybody, but decisions have been taken and now I do believe that justice will prevail," he said.

He said he believed in "human fairness" and that it was a "matter of perhaps another opportunity - of coming again and explaining . . . We shall do our best."

The tone of the interview was set when Begin was asked if this recent talks with President Carter could be counted as among the most difficult moments of his life, a remark he was quoted as having made.

"My friend, may I interrupt you for a while," he responded. "If I have to compare hours, days of my life in the past with a difficult conversation with the president of the United States and his advisers . . ."

The rest was left unsaid but a slight nod of the head encompassed the years of hiding from the British as the leader of what was then considered to be an extremist resistance group, the desperate struggle with the Arabs and Begin's own steadfast and uncompromising integrity toward his belief in what was right for the country in nearly 30 years of opposition politics.

His tone was subdued, lacking in rhetoric, but the impression given was of a man in control where outside pressures, domestic protests or even hints of palace coups would succeed in either changing his mind or loosening his grip.

"There may be ups and downs," he said. "When I was elected there was a very unfavorable opinion and two months later I was proclaimed as a man of peace throughout the world."

The alliance with Israel is a cornerstone of American policy and vice versa, he said, and this would not easily change.

Begin is prepared to continue to pursue peace with Egypt and the return of Defense Minister Ezer Weizman to Cairo next week was viewed as an encouraging sign.

He also is prepared to continue parallel discussions with the United States and, last Monday, he suggested to the Americans that Foreign Minister Moshe Dayan return to Washington to continue talks. He had not yet received an answer, he said.

But on the issues that divide him from the Egyptians and, in part, from the Americans he left no doubt that he is not prepared to make territorial compromises on the West Bank and Gaza Strip under any foreseeable circumstances. No Arab state had ever agreed to a meaningful territorial compromise on the West Bank either, Begin said.

Nor would he agree to add the words "on all fronts" to U.N. Resolution 242, which calls for Israel to withdraw from occupied territories - a condition Egyptian President Anwar Sadat has so far insisted upon if formal negotiations with Israel are to resume.

Begin explained his stand on the West Bank by saying that the presence of any potential enemy so close to the population centers of Israel would place Israel in "mortal danger" and that no security measures, guarantees, demilitarizations or anything else the world could suggest would make it otherwise.

"Demilitarization in the Sinai is a practical proposal," he said. "It is a desert. It can be checked. Any idea of demilitarization of part of the populated land [of the West Bank and Gaza] under foreign sovereignty would be a hoax because you can keep a gun in every garage. This is not a theoretical assumption. We have had experience from the past."

"And let us not forget the so-called PLO. If we should relinquish control of security and public order in Judea and Samaria and the Gaza Strip, the PLO would take over in no time and nothing would prevent them from doing so. This is the reality of the Middle East."

Thus, in Begin's view, his own plan of limited self-rule is the only fair resolution between autonomy for the Palestinian Jews for security.

"This is my absolute conviction from the point of view of our national security," he said. He has never hidden the fact that he believes in the right of the Jews to the land of Israel, which includes, in Begin's view, the West Bank and Gaza, but he recognizes taht there are other claims. His plan leaves the question of sovereignty open.

"Anyone who says that I base my approach to this issue only on our right [to the land] is absolutely mistaken. The right to the land and ouur basic issue of national security and existence are inseparably linked," he said.

Begin insisted, however, that his plan called for negotiations on the basis of U.N. Resolution 242, which Israel recognized and had never refuted. Israel balked at inserting language that was never written into 242, but there were many different interpretations of 242. Begin said the different directly as had been agreed upon when he and Sadat last met in Egypt on Christmas Day. The Egyptians were free to present any counterproposals they liked but, so far, the Israel peace plan was the only peace plan and in Begin's view it has not had its day in court.

Therefore, Israel was suggesting that Egypt resume the political and military committees that Begin and Sadat had agreed to, but the prime minister admitted that so far Weizman has not brought back any new proposals from Egypt.

He repeated his deeply felt sense of injustice over his certainty that the only reason American moral support of his peace plan had slackened was because the Egyptians had not accepted it.

As for Sadat, he resisted any personal criticism of the kind Sadat has leveled at him in recent days. But he did say that Sadat "creates the impression that he doesn't pay any attention to what I say."

Begin said that he had warned Sadat, in a public statement, even before the Egyptian president's historic trip to jerusalem that the two demands of a complete withdrawal to the lines of June 4, 1967, with minor adjustments and self-determination for the Palestinians, were completely unacceptable to Israel. To meet these demands would inevitably lead to a Palestinian state, Begin believes. Begin repeated this stand both during and after Sadat's Jerusalem trip. The constant repetition of these maximum demands left doubt that Sadat really wanted the kind of peace Israel could live with.

"When the other side repeats time and time again the same stand and actually says you must accept my point of view first - whatever comes later is a different story - then I suppose the norm of fairness would say that this is not the proper attitude.

"After President Sadat's visit to Jerusalem we made an intellectual and moral effort to produce a peace plan. It should be a fair basis for negotiation, but it hasn't been negotiation, but it hasn't been negotiated. This is the whole difficulty," Begin said.

The interview ended with an impression that as long as Begin is in control, and there is now no serious challenge to his authority, Israel will press on for a resumption of peace negotiations but will not change its stand simply because of the erosion of Israel's diplomatic position in the world. No one in Begin's long career has ever accused him of sacrificing his principles simply because they appeared impractical at the time and Begin firmly believes the right is on his side.