Shortly before the assassination of Bashir Gemayel, the 34-year-old Christian warlord elected to be Lebanon's next president, a senior U.S. diplomat in Beirut sought to explain the deep U.S. commitment to the tough, Christian Maronite leader.

"It is not as if this were a country of 3 million saints and Bashir was the only one around tainted by violence," the diplomat mused. "The reality is there were no better practical choices; in fact, there was no lack of worse ones."

Now, after Gemayel's death Sept. 14, followed by the massacre of Palestinian civilian refugees by his Christian followers and the installation of his weaker elder brother Amin as president, this troubled nation has been plunged into renewed uncertainty.

Amin Gemayel thus far has proven unable to command his dead brother's vengeful troops, who have vowed to remain an independent force in the country to ensure the execution of the policies of their mentor. The United States now finds itself deeply involved in Lebanon's affairs, with 1,200 Marines deployed around Beirut and special envoy Philip C. Habib engaged in a new round of difficult diplomacy. U.S. policy makers face a political void that they say could get more dangerous.

"We are reduced to hoping for the best," said one U.S. diplomat in Lebanon disconsolately, "while expecting the worst."

As they survey a Lebanese policy that only a month ago looked like one of the few U.S. diplomatic successes in this troubled region since the 1978 signing of the Camp David accords, American diplomats privately acknowledge underestimating the number, and the power, of the crosscurrents in Lebanese politics.

The problem in Washington's thinking on Lebanon was that it had sought to build a policy around one man -- Bashir Gemayel -- without sufficient consideration of the nation's political imponderables, its tribal hatreds, its history of civil wars and religious antagonisms.

It was just last month that U.S. policy makers were basking in the glow of the seeming success of Habib. The U.S.-negotiated evacuation from the capital of the Palestine Liberation Organization forces and Israel's accompanying promise to pull back from around the city had created a brief mood of optimism.

On Aug. 23, the Lebanese parliament elected Gemayel as president, a move that pleased the Israelis, who had backed him as a secret ally for seven years, as well as American officials, who in the past year had been carefully grooming him.

Because Bashir Gemayel was tough and determined to reunite his fragmented land, his aims -- if not the means he used in the past to achieve them -- coincided with U.S. policy goals.

After Bashir Gemayel paid a private visit to Washington in July 1981 to lobby for U.S. support of his Christian dream of uniting Lebanon, U.S. policy makers -- who had been leery of him because of his violent past -- appeared increasingly to believe he was the only man with the strength and drive to accomplish such a feat. He suddenly seemed to be taken seriously, and increasingly to be supported by Washington. Through him, the reasoning went, Lebanon's problems might be solved.

As hopes for Bashir Gemayel rose among U.S. officials, efforts were made to wean him away from the Israelis, to boost his own self-esteem and to clean up his political act, which until then had consisted of the bluster of a military chieftain.

The Central Intelligence Agency was sent in to set up an "information link" with his Lebanese Forces' intelligence units under the command of Elie Hobeika, the 28-year-old aide to Bashir, who, according to Lebanese sources, two weeks ago led militia units in the massacre at the Shatila and Sabra refugee camps. Sessions were also held for Gemayel with visiting U.S. congressmen and officials. Discreet but insistent advice was given on how he should behave and talk.

How specific Gemayel's grooming became was evident after he announced his candidacy for president in July. In a later private meeting, Habib wagged his finger at him and said, "Now you be sure and mention national reconciliation in your first speech. You hear?"

Because Gemayel proved a good pupil and because his electoral chances soared in the wake of Israel's invasion -- and undoubtedly because of the scarcity of other candidates -- U.S. officials seemed convinced that the "new" Gemayel was capable of unifying his country.

That Gemayel's base of support was the fanatic Christians he had forged into the Lebanese Forces militia after the 1975-76 civil war somehow was lost sight of. That Gemayel's own past was one of brutality and violence, part of the reason U.S. officials originally had shunned him, seemed to have become less important as he mouthed the rhetoric of national reconciliation that Americans wanted to hear.

But, in the wake of his death, the Habib plan -- the keystone of U.S. diplomacy in Lebanon -- collapsed with the Israeli decision to invade West Beirut. That in turn led to Israel's introduction of Gemayel's Christian militias into the Shatila and Sabra refugee camps for the killing that has made a mockery of U.S. guarantees reported by Lebanese officials to have been made concerning the safety of the Palestinians.

Because U.S. policy apparently went little deeper than confidence in Bashir Gemayel, that policy today is in disarray. "Despite the installation of a new president, we are still not even sure who Bashir's real heirs are today," said one concerned diplomat. "Not only don't we know who the real heirs are," he said, noting that the country is facing a new and explosive instability, "but we're not even sure of what policies they espouse."