"It is no longer the Anwar Sadat initiative," a chagrined Arab diplomat opposed to the Camp David summit said last Thursday as the Egyptian president swept out of town, tucking a draft peace treaty with Israel into his pocket. "Now it is the Jimmy Carter initiative. Nobody else can ever define all those unresolved points and evasions, and make the thing work."

President Carter's successful mediation at Camp David has done far more than transform relations between Egypt and Israel, an enormous accomplishment to begin with. His bold exercise in summitry has greatly expanded the already considerable American commitment to that region and has added to it an implicit U.S. promise to help protect the budding Egyptian-Israel alliance.

It has taken a war, five years of the gotiations, his epochal trip to Jerusalem and a determination to defy the rest of the Arab world, but Anwar Sadat at last has an American president where he has always wanted him - squarely in the middle, as committed to arguing an Arab case as an Israeli case and morally obliged to up the ante needed to keep Sadat, and the agreement afloat.

Carter's move on the Arab-Israeli chessboard - from being an early advocate of a "comprehensive" settlement that was to include a role for the Soviet Union to becoming the architect and eventual guarantor of what amounts to a separate Egyptian-Israeli peace treaty that slams the door on the Russians and on Palestinians who want an independent state - helps explain the trade-off that Sadat sudddenly accepted at Camp David.

Equally important was Sadat's obtaining from Israeli Prime Minister Menachem Begin an agreement on the full return of Egypt's Sinai peninsula, on relatively generous terms. But to do so, he had to sign a "framework" agreement on the West Bank and the Gaza Strip that falls far short of what Egypt had previously demanded.

The new U.S. role and the series of late, untidy lurches toward break-through in the final hours of the 13-day summit that ended a week ago have also opened a period of intense uncertainty in other key American relationships abroad. Carter is now weighing the inevitable costs of such a success, especially in Washington's ties to two of the most antipodal nations on the globe, the Soviet Union and Saudi Arabia.

Details now emerging from extensive post-mortems of the secret talks indicate that a shared assessment by Carter, Sadat and Begin of a growing Soviet and Cuban threat in Africa and the Red Sea region played a role in getting the movement needed for a peace treaty, especially from Sadat who had turned down a similar offer from Begin nine months ago.

In the nine months that have elapsed since Sadat turned down a proposal from Begin at Ismailia that was very similar to the one the Egyptian finally accepted at Camp David, he has seen the Russians creeping closer, expanding their presence in Ethiopia and in the Egyptian's view eliciting weak initial responses from Carter, who also appears to have significantly altered his assessment of Soviet intentions during that same period.

In a metaphorical sense. The Russian Menace occupied the fourth chair at Camp David. Carter reportedly was not shy in evoking the regional gains the Russians and "radicalism" would make if agreement was not reached at the summit, opening his first private talks with each leader on that somber note.

Begin and Sadat also took up Russian expansionism on their southern flank in a private talk, at Camp David. Begin said last week in an interview in New York, stressing that it was not the subject of negotiations. Since the signing of the Egyptian-Israeli peace framework, other Israeli officials have begun to talk privately about the ultimate possibilities of a security alignment of Israel and Egypt, along with other Arab countries that can be convinced that the Soviets are a greater menace to them than Israel.

A Russian Menace discussion also may well hold the key to getting Sadat's most valuable ally. Saudi Arabia, to go along reluctantly with an agreement that openly flouts the Saudis' one essential condition - that any final pact with Israel regain East Jerusalem for the Arabs.

The Saudi royal family is being moved into a position of having to decide in effect whether it fears the Russians more than it dislikes the results of Camp David. Secretary of State Cyrus R. Vance has been in Saudi Arabia taking the measure of that question, and a senior U.S. official let reporters know that when Vance lunched with Defense. Minister Sultan bin Abdel Aziz. Friday, Russian expansionism and not Camp David was the main topic of conversation.

White House aides seem convinced that the Saudis will go along and bring with them Jordan's King Hussein, who is essential to making the complex set of negotiations laid out for the West Bank and the Gaza Strip work.

Initial Jordanian and Saudi criticism, which was coordinated, is basically "rhetoric for the Arab world," one longtime Carter adviser said last week as Vance left on his Middle East mission. "We are confident they are open to reason. After all, what choice do they have other than ending up playing the Syrian-Iraqi game" of depending on the Soviet Union?

But that relaxed view is hotly disputed by some administration foreign policy specialists, and it ran into rough sledding yesterday as King Hussein canceled a scheduled trip to Washington and sharby criticized the Camp David agreements. King Hassan II of Morocco, who is close to Sadat but even closer to the Saudis, notably failed to give Sadat a public endorsement for the accords when Sadat left Rabat for Cairo.

As the surprise rush to Jerusalem did in November when Sadat launched his initiative of direct talks with Israel, the Egyptian leader's rapid turnaround during the last two days of the Camp David negotiation cur out the middle ground that the Saudis like to occupy in Arab polities. The finality of the peace agreement Sadat proposes to sign with Begin by Dec. 17 makes the choices available even more discomforting to the Saudis than those of last November.

That in turn could present difficult choices for Washington, where a debate has apparently begun in the administration on using U.S. pressure on the Saudis if their support becomes vital to the accords' survival.

"We don't have that much leverage to play hardball against a government that provides the U.S. with one out of every four barrels of U.S. imported oil," said one foreign policy specialist. "Sadat doesn't sit on top of 250 billion barrels of oil reserves.

A similar but much more intense debate raged in the administration for nearly two weeks after Sadat's Jerusalem trip and his call for a Cairo "preparatory" conference. In retrospect, that debate, which was the first important split between Carter's national security affairs adviser. Zbignicw Brzezinski, and the State Department, ended with results that buried the comprehensive approach the administration had previously taken, helped spark the serious Soviet-U.S. clashes last spring and summer and pointed straight toward the Camp David accords.

The State Department was deeply concerned about the impact of the move on relations with the Russians, the Saudis and other forces that in the "comprehensive" view needed to be neutralized or won over if a settlement was to work. The White House decided, however, that it had to endorse Sadat urgently, and Brzezinski designed a concept of "concentric circles" of negotiations to replace the Geneva conference, and the comprehensive approach.

The first circle called for a separate Egyptian-Israeli agreement. It was to be tied loosely to the second circle, which was to encompass much slower foursided negotiations over the West Bank and Gaza. The third circle involved the unlikely prospect of negotiations with Syria.

In the following months, the Carter administration clashed repeatedly with the Soviet Union over human rights, espionage and most particularly Russian and Cuban involvement in Africa. Sadat and the Saudis, convinced the Soviet Union has drawn up a master plan to encircle and overwhelm them, began to pumparms and money into countries the [WORD ILLEGIBLE] been targeted by the Russians.

In his triumphant appearance before a group of U.S. senators last week, Sadat reminded them of his stand against the Soviets, and said he hoped the United States would now agree to replace the weapons and equipment he has given to friendly African countries.

At about the same time. Begin was telling a House committee. "We must not forget Angola. Mozambique, Ethiopia. South Yemen and some others" that receive military supplies from the Russians. In the New York interview on Thursday, he confirmed that he had made a conditional offer to Carter to let the United States have bases in Israel if it would serve global security purposes.

Two factors seemed to loom largest in Sadat's decision to end his six-year campaign for a peace treaty with Israel at Camp David, only nine months after he had turned down a similar offer from Begin at Ismailia.

At Ismailia, Sadat did not have the American stamp that Carter's intimate involvement with every phrase gives to the two Camp David framework agreements. Nor did he have Begin's conditional agreement to knock down Israeli settlements in the Sinai peninsula; Begin conceded that point only grudgingly, and late, in the 13-day negotiating marathon.

After initial tense meetings with Begin at Camp David, Sadat stayed aloof from everyone except Carter. In effect, he got the president to argue major portions of the Egyptian case with Begin, a pattern that has continued since the agreements were signed and a dispute crupted between Begin and Carter over freezing Israeli settlements on the West Bank.

On Sept. 15, according to his own account. Sadat was on the verge of walking out of a deadlocked conference. He stayed only because of his faith in Carter, he told interviewers later, in a possible foreshadowing of his view of the president's role in Carrying out the agreements. Sadat's cabnet has described the accords to the Egyptian public as meeting Arab demands for the return of the territories occupied in 1967.

"Whenever I meet with Carter, and whenever we sit together, not more than a quarter of an hour is needed to solve and problem whatever its size," Sadat said of his near walkout, which he refused to descrie in detail.

The issue that caused his near walkout may have been the Sinai settlements dispute. Israel reportedly took it seriously enough to propose a territorial swap with Egypt to enable the settlements to stay. But Sadat refused to budge, and eventually won his major concession from Begin.

If it is realized, the Egyptian-Israeli peace treaty would be a de facto, comprehensive agreement in the sense that without Egypt the Arab world cannot wage a credible war against Israel.

For the residents of the West Bank and Gaza, Sadat did get a partial Israeli withdrawal, and Israeli pledge of due process of law which Sadat said here would lead to the release of "thousands of Palestinians who are now in Israeli detention camps and prisons," and a chance to elect an administrative council that will have full autonomy." But under the agreements, the autonomy does not apply to East Jerusalem, which Israel formally annexed in 1967.

The overlapping negotiating scheme for the West Bank, which resembles Brzezinski's concentric circles outline, is where the ambiguities and evasions are concentrated.

"Tha agreements are a new U.N. Resolution 242," said one diplomat referring to the deliberately vague document adopted after the 1967 war. "It exists so everybody could read into it what they want. The same is true here. If there is to be progress, it will be up to Carter to get his version accepted."

U.S. officials say that Sadat is likely to get increased food and other U.S. economic aid if he asks for it in the wake of the Camp David success. Unlike Henry A. Kissinger's Sinal II disengagement agreement, which opened up a new pipeline of military aid for Israel, long-term aid commitments do not seem to have been part of the Camp David agreements.

But in sponsoring the accords, Carter has mounted a major U.S. investment, whose size probably even he cannot estimate at this point, in Sadat and the fate of the Egyptian-Israeli peace treaty.

It is an ultimate kind of involvement that former secretary of state Kissinger touched on in comments to reporters in Rochester, N.Y., last week. While praising the accords, he said of Carter's direct involvement: "It is too risky. If something goes wrong, then you have nothing to fall back on."

But he said early hardline Arab statements would not affect the president's chances of maintaining the success of Camp David. "One cannot always tell in the Middle East where reality ends and epic poetry begins."