There is one essential new fact about the Middle East this weekend. The future of Lebanon is in the hands of President Hafez Assad of Syria. Whether Lebanon's official President, Amin Gemayel, has a political future will be determined in Damascus by Mr. Assad. What the relations of Lebanon will be with Israel will be determined in Damascus by Mr. Assad. Indeed, whether there will even be a Lebanon in the future will be determined in Damascus by Mr. Assad.
Washington's ability to influence Lebanon and its future drained away this week as United States Marines began their ''redeployment'' to the ships of the US Sixth Fleet offshore. Their role in the current story of Lebanon was finished. They could no longer influence the course of events there. Logically, and reasonably, they were allowed to go away.
The departure of the Marines was the denouement in a dramatic chapter in US foreign policy. It dates from Oct. 29 of last year when President Reagan gave formal approval to a radical change in US policy toward the Middle East. This was six days after the truck-bomb explosion that killed 241 US military servicemen.
The Marines had been in Lebanon for a year as part of a multinational peacekeeping force that had been sent in originally in the wake of the massacres of Palestinian refugees in Sabra and Shatila camps just south of Beirut. The original function was to separate Israeli armed forces from Lebanon and to try to rebuild a peace for Lebanon.
The role of the Marines was changed by National Security Council Decision Directive 111, which the President signed that day. The directive called for closer association with Israel. The presence of the Marines ashore, of supporting naval forces offshore, and military collaboration with Israel would, it was argued at that time, bolster the Gemayel regime in Lebanon and induce Syria to accept and agree to a mutual withdrawal of all foreign forces from Lebanon.
The rationale behind the change in policy was that ''a strong demonstration of US support for Israel was necessary to convince Syria that it could not win domination of Lebanon by military means.''
The effect was to make the US, and hence the US Marines, partisan supporters of the Gemayel faction in Lebanon, and hence opponents of the rival Shiite, Druze, and other factions seeking a larger share in the government of Lebanon. It also aligned the US openly with Israel and with Israel's interests in any future government of Lebanon.
Washington's ability to influence events in Lebanon has been going downhill ever since. The Marines were soon among the targets of Druze and Shiite militia units battling against the Gemayel regime. That military campaign reached its climax this week on Monday when the militia forces backed by Syria broke into west Beirut and cut off the Marines from the forces still under Gemayel control.
By the end of Monday's fighting nearly half of the Lebanese Army had gone over to the rebels. The rebels now control all of Lebanon except for the southern third still under Israeli occupation and the Maronite Christian enclave just north of Beirut.
Last October the policy of using Israel to help sustain the Gemayel regime was controversial inside the administration. Its major advocate was Lawrence Eagleburger, undersecretary of state for political affairs. His argument, well known around Washington, is that the closer the US is to Israel, the more amenable the Arabs are to Washington's wishes.
That argument was opposed by older State Department experts on the Middle East and by Secretary of Defense Caspar Weinberger. Joseph L. Sisco, who had been chief Middle East expert at the State Department during the Nixon, Ford, and Carter years, publicly urged the administration to seek its purposes in Lebanon through negotiation with President Assad of Syria.
President Assad was approached. Mr. Assad would negotiate, but made one condition. He insisted on abandonment of the agreement of May 17, 1983, which Secretary of State George Shultz had negotiated between Israel and the Gemayel regime in Lebanon. Mr. Shultz refused to scrap the May 17 agreement. Mr. Assad would do business on no other basis.
So we arrive at the end of a chapter in which the Eagleburger doctrine on how to influence Arabs has been tested and has failed. It could have worked, in theory, had both the US and Israel been willing to go to war with Syria. But Israel was not ready for another war. And Washington was not willing to send into Lebanon the kind of armed force that might have impressed the Syrians. They were not impressed by 1,600 marines. Their Army numbers about 400,000 and is newly resupplied with modern weapons by the Soviets.
The result is that the Reagan administration has sustained its first foreign policy failure. If it wishes to influence events in Lebanon now, it will have to go to Mr. Assad and accept his conditions. In practical terms, there is no other way.
Obviously, the May 17 Israel-Lebanon agreement is finished. The practical question is whether President Assad prefers to divide up Lebanon with Israel, or set up a new regime in Beirut that will be allowed to run all of Lebanon - with consideration for the wishes of Syria.