One year after the Israeli invasion of Lebanon, this hapless patchwork of a country remains the same battlefield of foreign forces it has been now for eight years and more than ever at the mercy of a heightened U.S.-Soviet rivalry in the Middle East.

War clouds once again are gathering over it. A year of intensive U.S. diplomacy belatedly has given birth to an agreement for the withdrawal of Israeli forces that contains the mechanism for the accord's own destruction--the approval of Syria, which is dead set against the deal.

"This accord has come far too late," said Saeb Salam, the highly NEWS ANALYSIS respected leader of Beirut's Sunni Moslem community who played a key mediating role in negotiations last summer for the withdrawal of Palestinian guerrillas from the capital. "The United States dragged on and on, and now everyone is turning against our agreement."

There is a deep sense of foreboding here that Lebanon's fate has fallen hostage to the rivalry of the two superpowers and that there never will be a withdrawal of all foreign forces unless Washington and Moscow can agree on steps to revive the stalemated attempts to reach a broader Middle East peace.

"The line of separation between the superpowers passes right through Lebanon. We are in the middle of the game," said Fady Hayek, spokesman of the Christian Lebanese Forces militia. "The dimensions of the conflict today are much greater than on June 6, 1982."

The expanding Soviet role in the conflict is directly linked to Moscow's support for Syria, which is attempting to check U.S. influence in the area and to establish its role as a key actor in the Middle East peace process.

For its part, Syria, the only Arab country to provide any military support to the Palestine Liberation Organization last summer, may now be cast in the ironic role of becoming its executioner, finishing the job the Israelis started to do through their invasion and siege of Beirut.

Syria is backing the radical factions presenting PLO leader Yasser Arafat with his most serious leadership challenge. His overthrow, if it comes, is likely to fragment the PLO into a dozen hopelessly squabbling factions.

But the biggest irony involving Syria is its position today. Despite the near destruction of its air force and the severe pounding of its army by the Israelis last summer, it is, with the help of its Soviet patrons, far stronger militarily than a year ago.

The loud Syrian rejection of the Israeli-Lebanese accord has catapulted Syria into the center of Arab and superpower diplomacy, a role it has long coveted and seems to be playing for all its worth and risks.

The Syrian reversal of fortunes in one year seems to confirm the observation of Lebanon's coordinator of the withdrawal negotiations with Israel, Ghassan Tueni, who remarked in an interview, "There are no political defeats attached to military defeats in the Arab world."

The lasting consequences of last summer's invasion for all the main actors--Israel, the PLO, Syria and Lebanon--have been slow in unfolding and continue to work themselves out on the complicated Arab-Israeli political checkerboard.

Israel has become seriously bogged down in the Lebanese quagmire, the PLO is facing disintegration and Lebanon seems more threatened by partition than ever.

In the long run, the most important fallout from the invasion for Arab, Israeli and international politics may well prove to be the destruction of the PLO, whose forces have been scattered throughout the Arab world.

"The most important factor of the invasion was depriving the PLO of an independent base," said Ihsan Hijazi, editor of the respected Middle East Reporter. "The rebellion happening now is a delayed reaction to the Beirut invasion."

The mood here on the anniversary of the Israeli invasion is a strange mixture of deep pessimism and flashes of optimism, the latter stirred by a rare demonstration of national unity in favor of the withdrawal accord with Israel.

"We proved we could deliver to the Americans and had enough authority to sign the agreement and remain in one piece," said Tueni.

"The accord helped to create a consensus around the withdrawal of foreign forces. We all want to get our friends out," he said, in a reference to the various Lebanese factions long aligned with one foreign power or another.

Hayek, the Lebanese Forces spokesman, agreed, noting that President Amin Gemayel had rallied the support of Moslems and Christians for the agreement in a show of unity not seen here, he said, since before 1970.

But Salam, the Beirut Sunni leader who masterminded two votes overwhelmingly in favor of the accord in parliament, warned that the consensus remained "fragile" and could be eroded before long by its nonapplication and Syrian intrigues inside Lebanon.

"After the invasion, there was a spell of moderation. Extremists in Lebanon and the Arab world were at their weakest point and disappearing," he said. "We asked the United States to use the opportunity, but they did not."

"Now the extremists are back again, he added. "They are opening the door wide to Soviet infiltration. The pendulum is swinging back. We will, of course, suffer, but you will suffer, too."

The Lebanese pessimism stems, too, from the general belief in the very real possibility of yet another war between Syria and Israel, again on Lebanese soil, and renewed fears of partition that seem to have become a permanent part of the tortured Lebanese national psyche after eight years of civil war and turmoil.

There is a feeling shared by Moslems and Christians that Israel is in an untenable position in Lebanon and will not stand by much longer without striking out in some direction.

On the one hand, it is faced with a war of attrition against its occupying troops in southern Lebanon mounted by increasingly aggressive Palestinian guerrillas, Lebanese leftists and Shiite fundamentalists. On the other, it confronts a rapidly expanding Syrian military might.

The Soviets have done more than simply replace Syrian losses in planes, tanks and missiles from last summer. They have provided Syria with sophisticated SA5 antiaircraft missiles, manned and defended by their own experts, and more than doubled the number of military advisers to around 5,000, some of whom have been spotted with Syrian units stationed in Lebanon's eastern Bekaa Valley.

"The Israelis have a national phobia about Arab military power," said Hayek. "They need to act before it becomes very expensive or impossible for them to do so."

There are many theories about when and how Israel might strike. One of them being widely discussed here is that it will not attack Syrian and Palestinian forces stretched all through the Bekaa, as everyone expects, but instead try to penetrate directly into Syria close to Damascus, hold a bulge of land and then use it in negotiations to force a Syrian withdrawal from Lebanon.

Whatever action Israel might take against Syria, Lebanese seem to look upon a unilateral, partial Israeli withdrawal to the Awali River just north of Sidon--now being debated in Israel--as a kind of kiss of death on all remaining hopes of reuniting their fragmented nation.

They fear that this would only ensure the detachment of southern Lebanon from Beirut's authority, intensify Syria's determination to stay in the Bekaa and encourage the PLO to take over northern Lebanon.

Meanwhile, despite the destruction of the U.S. Embassy April 18 and several other bombings, restoring calm and unity to the capital is probably Gemayel's major accomplishment to date and the biggest change from last June.

The Lebanese Army, now being rebuilt from top to bottom, has established its presence throughout the city. Christian East and predominantly Moslem West Beirut have been reunited physically, if still not altogether psychologically.

"This capital is in a distinctly better position than it was a year ago," the British ambassador, Sir David Roberts, said in an interview. "One year ago, there were roadblocks by various militias and hoodlums everywhere. You never knew what you would meet around the next corner. Now you know. The Lebanese Army."

But the rest of the country remains outside the central authority. The only possible extension of this authority in the next few months would be into the Chouf Mountains, if the Israelis withdraw there.

But President Gemayel and Lebanese Army commanders say that even this will not be possible unless there is an accord to end the fighting there between Christian and Druze militias. Negotiations are under way, yet leaders of both militias seem skeptical that an agreement, even if reached, can be implemented because of the blood debts accumulated this year among families on both sides.

Thus, the prospects for putting the pieces of this nation together do not appear much better today than they did a year ago or any brighter than getting a Syrian agreement to withdraw in the next few months.