Behind the tragic drama of war in Lebanon lies a complex of fears and tensions that puts the entire Middle East -- Israel, the Arabs, and Iran -- on edge.
First task of US diplomacy, as seen by experts here, is to prevent those tensions from exploding into a general Mideast war, ignited by the Israeli attack on Palestinian strongholds in Lebanon.
American experts find hope in the fact that neither Israel nor Syria, which has peace-keeping troops in Lebanon, shows any desire to tangle with each other at this time.
''The chances are pretty good that the war in Lebanon will be containable'' and that the worst -- a wider Arab-Israeli conflict -- can be avoided, says William B. Quandt of the Brookings Institution,
Israel's current plans, says former Undersecretary of State Joseph J. Sisco, include neither war with Syria nor permanent retention of Lebanese territory.
A cease-fire, experts say, will not in itself dissolve the root cause of chronic instability along the Lebanese-Israeli frontier. Permanent calm can only come, specialists agree, when the future of the uprooted Palestinian people is assured in a way acceptable to Arabs and Israel -- a prospect not yet in view.
Meanwhile, experts cite an array of reasons why no Middle East leader currently wants to shepherd his people into another round of Arab-Israeli war.
''For one thing,'' says Mr. Sisco, ''the Israeli-Egyptian peace treaty has fundamentally altered the military balance of power, so that an all-out Arab war against Israel is impossible.''
President Hosni Mubarak of Egypt, having just regained control of the Sinai Peninsula from Israel, in no way wants to jeopardize that gain, specialists agree.
The ultimate goal of Israeli Prime Minister Menachem Begin may well be, some US officials say, to annex both the West Bank and Gaza, occupied by Israel since 1967.
At present, however, the more limited aim of the Israeli invasion of Lebanon is thought to be exactly what Mr. Begin proclaims -- to clear the armed forces of the Palestine Liberation Organization (PLO) out of southern Lebanon, beyond rocket and artillery range of Israeli towns and settlements in northern Galilee.
Syrian President Hafez Assad, preoccupied by internal political dissent and by border tensions with Iraq to the east, is unlikely to challenge Israel's superior military machine, most experts agree.
Sisco, speaking earlier on a Voice of America (VOA) broadcast, perceives ''a political pincer move by Syria and Iran to oust President Saddam Hussein from power in Iraq.''
Seen in this context, a war with Israel not only might weaken Mr. Assad militarily and politically, but it would distract him from his goal of replacing Saddam Hussein with an Iraqi leader more to Damascus' liking.
Saudi Arabia's royal House of Saud, meanwhile, and the sheikhs of the Persian Gulf emirates -- all Sunni Moslems -- fear a campaign by Iran's Ayatollah Khomeini to export his Shiite revolution to their territories.
King Hussein of Jordan, who ousted the PLO from his kingdom in a bloody struggle more than a decade ago, neither desires nor is able to help Palestinians in Lebanon, experts agree.
Against this background, the interests of all concerned -- except possibly officials of the PLO -- would appear to be to localize the war in Lebanon and to bring the fighting to an early end. Desperate leaders of the PLO, reeling toward military defeat, might try to provoke Israel into military actions that would, willy-nilly, engage Syrian forces stationed in Lebanon.
Immediate focus of American concern is restoration of the tattered cease-fire , through the mediation efforts of President Reagan's special envoy, Philip Habib.
This will be no easy task, for the Israelis demand a stronger cease-fire than that which resulted after the 1978 Israeli incursion into Lebanon. This time around, Israeli officials insist, they intend to attain and enforce a cordon sanitaire -- empty of the PLO -- from Israel's frontier up to and beyond Lebanon's Litani River.
Still unclear, as the fighting rages, is what kind of guarantees the US and the international community could offer that might induce Israel to relinquish its hard-fought gains -- including the port city of Tyre and the strategic fortress Beaufort Castle -- and withdraw its army from Lebanon.
Assuming a durable cease-fire can be achieved, says Sisco, the United States should ''play a critical role'' in revitalizing broader peace initiatives in the Middle East.
Already the Reagan administration has toned down its earlier emphasis on forging a ''strategic consensus'' against Soviet influence in the area, involving the US, Israel, and moderate Arab regimes, notably Egypt, Jordan, Saudi Arabia, and the Gulf sheikhdoms.
This effort foundered on the refusal of King Hussein and the Saudis to identify themselves with a policy initiative that explicitly included Israel.