The diplomatic countdown leading to the last and most crucial phase of Israel's staged withdrawal from the occupied Sinai Peninsula may begin within 10 days. That is when Foreign Minister Yitzhak Shamir is scheduled to arrive in Washington for talks with US Secretary of State Alexander Haig.
The Israeli diplomat is expected to ask his American counterpart to help organize a multinational force to supervise the strategic Israeli frontier zone from rafah on the Mediterranean Sea to Eliat on the Gulf of Aqaba.
Former President Jimmy Carter formally committed the United States to the establishment of such a force in the event United Nations machinery did not make itself available. It did not, just as the UN refrained from giving its blessing to the US-backed Egyptian-Israeli peace treaty.
Authoritative Israeli quarters are frank about the implications. "Without an agreement on an international police unit, we cannot see how we can complete our withdrawal from the rest of Sinai," a senior official said.
It did not matter to him whether American troops would be included in the unit or not, nor did he have any objection to the multinationals and/or a US contingent using Israeli's three Air Force bases in eastern Sinai that are to be evacuated.
The US has had a peacekeeping contingent in Sinai since 1974 when the Israeli-Egyptian disengagement process initiated by former Secretary of State Henry Kissinger led to the establishment of the Sinai Field Mission. Since the peace treaty, the field mission has expanded as a temporary referee watching Israel's staged withdrawal and the observance by both sides of the treaty's military protocol on arms and troop limitations.
Aircraft were added to the mission's resources and the entire contingent with its logistical base could become the nucleus for the future multinational force.
Under the peace treaty, Israeli military personnel and equipment must quit Sinai by March 26, 1982.
Outlining some of the responsibilities of the multinational force, presumably in keeping with the points likely to be made by Foreign Minister Shamir to Secretary Haig, the official stressed its role at Sharm el Sheikh, the fortress controlling access through the Strait of Tiran to the Gulf of Aqaba from the Red Sea.
"It will have to assure free passage for ships of all countries," he said, alluding to Egypt's 1967 blockade, as a result of which Israel launced the preemptive strike that opened the "six-day war" and led to its seizure of Sinai, the West Bank, Gaza Strip, and golan Heights from Egypt, Jordan, and Syria.
Facilities, vehicles, and procedures also would have to be arranged for the forced to patrol the land border stretching nearly 150 miles across a landscape of shifting sand dunes and craggy mountains.
These and other sensitive surveillance assignments cannot be given in a last-minute, ad hoc improvisation. And American failure to set up a force whose makeup and technical competence would satisfy Israeli as well as Egyptian military men could perpetuate the status quo in Sinai -- a cease-fire line extending from El Arish to Ras Muhammad, with the three Air Force bases remaining at Israel's disposal.
There is no dodging the US commitment, according to analysts here. It is spelled out in identical letters from Mr. Carter to President Anwar Sadat and Prime Minister Menachem Begin dated March 26, 1979, and appended to the Egypt-Israel peace treaty signed on the same date.
"If the Security Council fails to establish and maintain the arrangements called for in the treaty," the letters say, "the President will be prepared to take those steps necessary to ensure the establishment and maintenance of an acceptable alternative multinational force."
This was stated after a reiteration of the treaty's provisio for the "permanent stationing of United Nations personnel in the designated limited force zone" as something that "can and should be implemented by the UN Security Council."
Strictly speaking, therefore, it will be up to President Ronald Reagan to fulfill this commitment.
Had the Soviet Union not turned its back on the Egypt- Israel peace treaty, tacitly casting a veto by publicizing Kremlin disapproval of the accord, Mr. Reagan might have been off the hook. But the UN will have no part in the implementation or supervision of the peace treaty's provisions.